ABBREVIATIONS OF WORKS CITED
NHD: Zubiri, Xavier: Naturaleza Historia Dios, Editora Nacional, Madrid, 1942, 1963 (5th ed.).
SE: Zubiri, Xavier: Sobre la esencia, Sociedad de Estudios y Publicaciones, Madrid, 1963.
HOM: Varii: Homenaje a Xavier Zubiri, Editorial Moneda y Crédito, Madrid, 1970.
REAL: Realitas I: Seminario Xavier Zubiri: Trabajos 1972-1973, Sociedad de Estudios y Publicaciones, Madrid, 1974.
 Zubiri, it has been claimed, has achieved not only a new philosophy, but a new idea of philosophy as well. Like all such claims this, too, savors of a certain extravagance and must be greeted with a certain reserve, a reserve, however, which is sensitive to the element of truth which this claim undoubtedly contains. Zubiri's claim to attentive audience does not derive primarily from the novelty of his philosophical doctrine or of his idea of philosophy, although both exhibit this quality. Novelty, of itself, generates no obligation and ministers only to curiosity. Zubiri's claim has a far more solid and compelling basis, namely, the manner in which he addresses the most fundamental problem of western philosophy and the powerful personal insights he brings to its illumination. It is this continuity with and enrichment of the old and the abiding which gives significance to what is new in his thought.
This most ancient and fundamental problem of western thought is simplicity itself; indeed, overwhelming in its simplicity but far more so in the profundity which this simplicity conceals. This problem is, in Zubiri's own words "el esfuerzo por entender el último de las cosas," the effort to comprehend the first principle of things. This principle, whatever its character, is what is really real in itself and what, in turn, through the communication of itself, its "dar de sí," is the source of all else that is real and that really is. The quest for this principle is philosophy. Its presence is the most distinguishing mark of western culture. The form which this quest has taken, the methods by which it has been approached and the resolutions which have been offered for it impart to that culture its distinctive quality  and character. This is the problem which Zubiri addresses. In this address he has exhibited great perspicacity and inventiveness and has given evidence of powerful personal insight. He has elaborated the classical formulations of the problem in significant fashion and has drawn from the ancient and classical resolutions of it dimensions hitherto latent. His claim to audience, consequently, stems neither from antiquity nor from novelty taken in abstraction but from that penetration of the two of which the fabric of history is woven.
Particular force and interest accrues to Zubiri's claim from the fact that in all that is novel in his idea of philosophy and in his philosophical doctrine the most ancient resolution of that fundamental problem receives fresh vindication. To this most ancient view of "el último de las cosas" the venerable term 'realism' has consistently been assigned. Behind this historical term lies a complex insight. This is the insight that whatever "el último"-the principle of all that is real and which itself is most real, because real "de suyo" and as "suyo" -may prove to be, it is a principle independent of man and of man's knowledge of it and must indeed be recognized as the principle of the reality of man - of the reality that he is and of the reality of what he is. At the same time, and through the agency of what man is in his reality, the reality of that principle is reciprocally realized. This is the insight, expressed in roughest outline, which historically first signalizes the presence of philosophy in western thought and first establishes western culture as philosophical in its most profound dimension. This is also the position to the defense and deepening of which Zubiri, drawing upon, and in no wise rejecting or opposing, the powerful insights and formidable resources of modern and contemporary thought, returns. Under this aspect his thought constitutes, not metaphorically, but quite literally, a ricorso  in the Vichian sense: the great circle in which the human spirit returns to its source, not however, there to be lost in a featureless identity, but to re-emerge enriched and illuminated.
This fresh and deeper penetration of the classical realism of the west, through the most sophisticated agencies of modern and contemporary thought, is a position at which Zubiri arrived by no easy route. On the contrary, his has been a long and arduous journey, to the term of which he has not even yet finally arrived. As in every significant pilgrimage of the human spirit, this journey's end, as the poet says, is in its beginning. The work of Zubiri here presented for the first time in English translation Sobre la esencia, On Essence is recognized as the culmination, though not the termination, of that long and arduous pilgrimage which has been his life. By this same principle, consequently, the appreciation of this work, too, must lie in its beginning.
It has become a commonplace, after the numerous reiterations of Croce and other critics, that the extra-philosophical life of the philosopher, like the extra-poetic life of the poet, is of little interest. Yet it does help in the comprehension of a philosopher's thought to know something of his life and particular circumstances, especially as, in Zubiri's own view (we hear an echo of Ortega), philosophy is always projected from a "situation." A few indications of Zubiri's life and career will not, therefore, be amiss.
Xavier Zubiri was born in San Sebastián in 1898. After preparatory studies he enrolled in the University of Madrid where his intellectual interests quickly found their focus in philosophy and theology. To a great extent, no doubt, this was due to the influence of the masters it was his good fortune to encounter, above all Don Ángel Amor Ruibal, Don Garcia Morente, Don Juan Zaragüeta and Don José Ortega y Gasset. Each of these men presented  him with a model of the intellectual life marked by its own special virtues but all characterized by a common trait, absolute integrity. If, however, the specific accents of the influence of each on Zubiri's thought may be noted, it may be said that the example of the weighty Zaragüeta more than all others placed his feet firmly in the paths of classical western thought while that of the adventurous Ortega, always open to the new and unexplored, urged him into the wider fields of modern speculation.
His studies at Madrid were varied by periods of residence at the University of Louvain and the Gregorian University at Rome. The former was the center of the new scholastic and Thomistic movement which had been inspired by Pope Leo XIII. This center was under the aegis of the magnetic and enlightened figure of Cardinal Mercier and was attracting the best Catholic intellectual talent from many parts of Europe and abroad. The Gregorian, of course-the lineal descendent and continuator of the Collegio Romano of Counterreformation fame from which much of the creative energy of that movement had flowed-was a bastion of solid traditional philosophical and theological learning, while at the same time programmatically sensitive  and responsive to the currents of modern thought. At the Gregorian, in nineteen hundred and twenty, Zubiri received the doctorate in theology. Thence he returned to Madrid to receive the equivalent degree in philosophy the following year. His thesis, published in 1923 and entitled Ensayo de una teoría fenomenológica del juicio already projects the range and quality of his mind.
Zubiri entered upon a university career and the year nineteen hundred and twenty-six finds him occupying the chair of the history of philosophy at the University of Madrid. His more urgent interest, however, lay in the enrichment of his own culture and in extending contacts between philosophical thought in Spain and the dominant movements in other countries of Europe; he was also much concerned to extend and strengthen his own bases in the physical, biological and humane disciplines. The years 1928-1931, consequently, found him resuming the role of student and traveling to various centers of learning in Europe, seeking out masters of thought in various areas of study. His interests were and remain encyclopedic. These years find him studying classical philology with Werner Jaeger, philosophy with Husserl and Heidegger, theoretical physics with de Broglie and Schrödinger, biology with von Geluchten, Spemann and Goldschmidt, mathematics with Rey-Pastor, La Vallée-Poussin and Zermelo. Despite this diversity of interests, Zubiri's real talent is not encyclopedic. On the contrary, it is highly concentrative and all of these interests served only to minister to his sustained central concern - philosophy and philosophical theology. The years 1936-1939 find Zubiri in Paris, exercising the dual role which defines his life: teacher and scholar. While offering courses at the Catholic Institute in Paris, he is engaged in the study of oriental languages  with Deimel, Benveniste, Labat, Dhorme and others at the Sorbonne. In the subsequent years 1940-1942 he occupied the chair of the history of philosophy at the University of Barcelona.
The year 1943 must be accounted crucial in Zubiri's career. Never an academic man in the narrower sense of the term, in this year he left the university to strike out on an original program of research and teaching on a model all his own. This led to the creation of his own particular organ: the cursos, which he has presented and continues to present since that date. The cursos, even more than the treatise or essay, must be recognized as Zubiri's personal and original mode of expression and communication. In them, even more than in his books and publications in learned journals, is to be found the living movement, the vital rhythm as well as the weighty insights of his thought. To have assisted at them, even occasionally, as the present writer has, proves a memorable experience. Their texture is dense, but lucid. Above all it is the sense of direct participation in the quest of truth, in the immediate communication with the deep personal quest of a powerful intellect informing a personality of unshakable devotion and integrity that most impresses one; this and the particular didactic style, in which the auditor is not passive, but rather drawn directly into the active intellectual emprise in which the lecturer is so manifestly absorbed. The full force and quality of Zubiri's thought, as has been suggested, is to be found in these cursos, even more perhaps than in his formal publications.
Ignacio Ellacuria has suggested that Zubiri's activity falls into three major phases. The first phase runs from the appearance of his first published writings to the establishment of his program of private cursos, embracing the twenty years, that is to say, from about 1923 until 1942. The second phase covers the period from the establishment of the cursos until the appearance in 1962 of Sobre la esencia when the third and present on-going phase was initiated. Prior to this last event, Zubiri's major, though not, of course, exclusive, preocupation had been with  the "idea" of philosophy; more precisely, both the idea as represented by the actuality of its history and the idea as an "ought," an ideal. His concern had been to determine what philosophy has been in its historical actuality, as a basic dimension (indeed, in a phrase he will come to use more and more, as the constituting principle), of western culture. Even more, he had been concerned to identify philosophy in its "idea": that is, the kind of knowledge in which it ought to reside, the kind of attitudes and operations it demanded of the "philosopher" and above all the way in which this kind of knowledge and these operations affect the philosopher in his existence and especially in the state of being and the relation to reality in which philosophy establishes him. For it seems clear that if philosophy is not effective in this way-that is, if it does not establish its subject, the philosopher, in a particular, and is a particularly real, relation to reality, and to reality, not in its mere factuality but "principially" (the term is his own), in its effective principle - all that can be said of philosophy is what St. Paul said of faith without charity-it is sounding brass and tinkling cymbals. The second phase is marked by the program of the cursos. In these cursos as we have noted much of the most valuable material for an understanding of Zubiri's thought is to be found; indeed, they possess a unique value, because here are to be seen not merely the results of his thought but its process. Were the present essay concerned with his total system and not intended to serve principally as an introduction to the English translation of Sobre la esencia the cursos would unquestionably serve as a principal source.
The appearance of Sobre la esencia, nevertheless, must be considered the major event in Zubiri's work; this treatise (and its similarity to the "tractatus" of the second scholasticism has been noted) does two things: it brings to focus certain basic strains in the cursos of the preceding years and it opens a new constructive phase of his activity. In this work and henceforward his chief concern will not be with the idea of philosophy, historical or normative; his purpose will now be effectively to do philosophy (as a current English phrase has it); that is, to construct his own philosophy ("cada filósofo  ha construido su propia filosofía" as that "sistema de pensamiento unitario y deliberadamente organizado" which philosophy must always be. Our concern at this point will be to identify the salient points of the earlier phases during which the concern was principally with the "idea" of philosophy for it is this "idea" which he will seek to realize in his construction of philosophy in Sobre la esencia.
In these earlier studies Zubiri approaches the "idea" of philosophy along three paths: first, by way of the analysis of what it is to "philosophize" as an attitude and an activity peculiar to man; second, by way of a contrast between philosophy as a form of knowledge and science in the modem understanding of that term; finally, by way of the identification and characterization of philosophy by the "object" of philosophy.
Zubiri introduces his reflections on the idea of philosophy with the observation, noted earlier by Hegel, that the history of philosophy is the history of the idea of philosophy. What this history teaches, in the first place, is that the "content" of philosophy, the "doxai" of Diogenes Laertius and even their systemization have but a secondary and conditioned importance. These remain but an inert mass, without significance or life, providing merely matter for scholastic erudition, unless grasped in their creative and generative principle; it is the latter which truly concerns the quest for the idea of philosophy: "el principio que la mueve." The characteristics which Zubiri attributes to this generative activity of philosophy are striking; here we can but touch on a few salient features. In this activity the philosopher seeks "recibir original e indeformada, ante su mente, la  realidad del mundo y de la vida humana"; it is not an "occupation" of man, that is an activity which does not affect him as a subject, in his radical relation to being, to reality. Instead "es un modo fundamental de su existencia intelectual." All philosophy is executed or carried on from a "situation": "como todo hacer verdadero, es (la filosofía) una operación concreta, ejecutada desde una situación." This "situationism" does not, however, give license to subjective philosophical lyricisms. The process of philosophy is governed not by such subjective movements but by the dynamism of reality itself which is actualized in the mind of the philosopher. Reality holds primacy over philosophy, and philosophy holds primacy over the philosopher: "no es la filosofía obra del filósofo, sino el filósofo obra de la filosofía." The process by which this subjectivity of the man in situation is transcended is the inner movement of philosophy. This movement cannot, however, be characterized or formalized a priori. Philosophy can never be turned into a technic by being formulated into rules which can be applied universally by all manners of men. It always remains the work of the individual and the philosophy to which he gives expression is his philosophy. "La idea de la filosofía es distinta en cada filósofo porque cada filósofo ha construido su propia filosofía." Finally, the philosopher in his individuality cannot construct his philosophy a priori, by any plan or rule; it can only be the result of an inner movement of his own soul of which his philosophy is the expression. The philosopher is a philosopher only in view of the philosophy which he has constructed and he is a philosopher of a certain character because of the character of the philosophy which has issued from the inner movement of his own soul. "Ante una filosofía ya madura, y precisamente ante ella, es cuando resulta no sólo posible, sino forzoso preguntarnos hasta qué, punto y en qué forma responde a su propio concepto." Nevertheless, despite all of these highly personalistic dimensions of philosophy and the process of philosophizing, philosophy in its ultimate character is  one; the reason lies in the fact that in all philosophers and in all philosophies the ruling principle is reality which is not many but one; the one which all seek in a diversity of ways from a diversity of situations but with a unity of ultimate orientation.
The principle of philosophy, consequently, that on which its idea depends, cannot be an "objective," i.e., impersonal and quasi-mechanical principle, like the "method" of certain figures in the history of philosophy, involving no personal commitment or transformation of the philosophizing subject. The result of the operation or application of such a method would be a "fact," "hecho," a product and would always be an "other" to the philosopher himself, something alien and alienated. Philosophy is never a fact, "hecho," but rather "es cosa que ha de fabricarse por un esfuerzo personal." Even under the aspect of system-and philosophy must eventually be system-the aspect under which it might seem most "impersonal" and "objective," philosophy is unauthentic unless the systematization itself is the result of an authentically philosophical effort. This insistence on the personal dimension of the philosophical activity raises immediately the question of its intrinsic character and, less directly, the question of the transformation which the activity of philosophy and the achievement of its end effects in the philosophizing subject. Both questions receive an initial but determining response in Zubiri's formula already noted: "recibir original e indeformada ante su mente la realidad del mundo y de la vida humana." The presence of reality "original and undeformed" to the personal consciousness of the philosophizing subject clearly cannot leave that subject unaffected in its status in existence and in being. Indeed, the ultimate impact of philosophy under this most important aspect is upon this personal transformation of the quality of the act of existence of the philosophizing subject. And since it is reality "original and undeformed" which works this transformation, the transformation itself cannot be other than in the direction of the revelation of the reality of the subject and its establishment in that reality.
These observations must not, as suggested, lead one to conclude overhastily that philosophy is an entirely "subjective" operation. Such  a view would be in obvious contrast to the view that philosophy places "realidad original e indeformada" before the subject; that reality can by no stretch of the imagination be constricted to the limits of subjectivity. Quite the contrary, it is not the limits of subjectivity which define reality, but philosophy, reality, which defines and establishes the philosopher as subject. He is established in his character of philosopher by philosophy and not conversely: "no es la filosofía obra del filósofo, sino el fil6sofo obra de la filosofía en el sentido ... de que no somos nosotros los que poseemos la verdad sino de que nosotros somos los poseidos por la verdad."
Thus at the heart of the "idea" of philosophy innests this reciprocity: philosophy is the result only of a personal effort of the philosopher not, however, in the sense that philosophy is the work of the philosopher but rather in the sense that the philosopher is the creation of philosophy, of reality and of truth as these take possession of him. Paradoxical as this may seem, it is essential, for Zubiri, to the understanding of philosophy and it is this reciprocity which supplies the inner dynamic of the process. Finally, this view of the "idea" of philosophy has a direct reference to the interpretation of Zubiri's own thought, for it provides a most important entry into the structure of his own system.
Philosophy establishes the philosopher in a particular state of being and existence, his "estar siendo." The condition of actual existence, "estar siendo," in which philosophy establishes the subject is essentially a condition of knowing, of knowledge. The quality of this knowledge obviously demands clarification and the path by which this clarification is best achieved is by a contrast between it and the concept of science especially as the latter takes form in modern thought.
This is the path which Zubiri takes and he concretizes this process in a contrast between the Greek notion of epistemé and the modern conception of science. His position is complex. It is no part of his concern (as happens in the case of certain other philosophers concerned to vindicate the specific, even privileged, station of philosophy as knowledge) to denigrate the modern conception of science. On the contrary, it is his view, as will be seen more clearly, that science is  essential to culture and-even more importantly, in this conjunction-to the project of philosophy; he makes every effort in the constructive phases of his thought to incorporate modern science, materially and methodologically, into its systematization. At the same time, he is concerned to keep these kinds of knowledge-philosophy (epistemé), and science carefully apart; the distinction between them, however, must be dialectical and not abstract. If released into abstract autonomy, these kinds of knowledge will lead to distinct concepts of reality, the one contradictory to and nugatory of the other. The real task is to identify the character of each as authentic knowledge, while locating them within the one reality, since the real can only be, transcendentally, one. The character of this difference-and at the same time the authenticity of each - may be exemplified and illuminated by a comparison of the manner in which science and epistemé respectively address common points of issue.
The first of these points of issue is movement and phenomenon. For science, in Zubiri's view, movement is the passage from one place to another, the course of which must be plotted by the correlation of a number of factors. For epistemé, what is important in movement is the mobile, in its condition as a changeable thing, internally unstable, and movement as a mode of being (ser) in relation to the non-being (no-ser) which it implies. For science, according to Zubiri, the phenomenon is a real event (acontecimiento) whose temporal and spatial area must be circumscribed by precise measurements and which further involves an observer, a measurer: e.g., the equations of modern physics as anticipations of observables. For epistemé, by contrast, what is important about the phenomenon in its manifestation is not the observer to whom it appears but that which appears, the "ens phenomenale" the thing itself which appears in the appearance.
Finally the respective address of epistemé and modern science to nature may define these differences even more clearly. Both seek the nature of things, but the idea of nature in the case of each differs. For science, nature is the system of laws, the norm of variations, the mathematical determination of phenomena that vary as, for example,  in Galileo, or the distribution of observables, as in the new physics. For epistemé, nature means movement, actual or virtual, emerging from the very depths of the being which moves-the principle of movement, a movement viewed from being (ser) and from causes.
It must be concluded then that science and epistemé, philosophy, are two different kinds of knowing. Both seek a "because," thanks to which we may know the inexorable necessity which penetrates reality. The "because" of science, however, is a "how".; that of epistemé is a "why." Zubiri sums up the difference by saying "La ciencia trata de averiguar dónde, cuándo y cómo se presentan los fenómenos. La epistemé trata de averiguar que han de ser las cosas que así se manifiestan en el mundo."
This difference as knowledge, however, finds its full impact in the fact that it is the idea of reality proper to each which inevitably differs as well as their ideas of what things are and of what the universe is, reality in the sense of what makes things what they are and what makes them that they are. For epistemé, things are not many notes united together, but a unity which is diversified in its notes. According as we place ourselves on the side of unity or on that of diversity we see what the thing is in itself and what it is for the observer. Science tends to believe that things are clusters of notes more or less united in their appearances and does not focus on the unity from which the notes are diversified.
While these different modes of knowing may be considered two valid approaches to things, equally authentic, they cannot, by this mere consideration, be thought to be of equal force. The point of view of science which seeks "la unidad...en la totalidad de los fenómenos es su conexión objetiva la ley" which totality is in turn interpreted as the totality of objective experience, is a great gain. It is not, however, at the same level, in the order of knowledge, as philosophy for which "el problema de la realidad de las cosas es esencialmente el problema de lo que ellas son y no simplemente de las condiciones intramundanas o transcendentalmente mundanas de su acontecer." It is to the former problem that philosophy, differently from science,  addresses itself and it is this difference which establishes its higher status as knowledge.
It would be easiest, in dealing with the "object" of philosophy, if philosophy might simply be said to have an "other" or different object than that of science; the different objects might then be characterized by different notes but in an identical idiom. But this is not the case because the relations of the one, science, to its object and that of the other, philosophy to its object, are completely inverse. Science has its object given before it and proceeds to illuminate and explore it. Philosophy, by contrast "...no se sabe cuál es su objeto. Determinar el objeto de la filosofía es la primera de sus tareas y el esclarecerlo debidamente es su coronación." In Zubiri's own words: "La filosofía tiene que ser, ante todo, una perenne reivindicación de su objeto (llamémoslo así) ... Mientras la ciencia versa sobre un objeto que ya se tiene con claridad, la filosofía es el esfuerzo por la progresiva constitución intelectual de su propio objeto, la violencia por sacarlo de su constitutive latencia a una efectiva patencia."
This observation does not imply for Zubiri, as an analogous observation did for the idealists, that philosophy must create its object. The intellectual anguish (angostura) which oppresses the philosopher and from which he tries to escape by seeking the proper object of philosophy does not lead him to the fantastic evocation of new objects; rather, it leads him to the discovery and possession of something which is not a concrete object, but which, nevertheless, has its own consistency. Some might conclude from this that philosophy is a useless quest, doomed to failure. Zubiri does not see it so. Philosophy cannot be thought useless because even its failure, if failure it prove to be, would tell us more of reality than the ostensible successes of the sciences.
What can be said with certainty is that man, in all his stances, is orientated toward reality. His first orientation toward reality is toward reality as realized in real things and over these he deploys his sciences. There is always the danger of conceiving philosophy as another of these sciences-a confusion the consequences of which must prove disastrous. Philosophy-the philosophic quest-is born when  man perceives that while reality itself can be approached only by way of things that are real, it cannot be equated without residue with those things so that the character of reality, even as it is the principle of the reality of those things as real, does not make itself patent to us in the same form or manner as do the real things which we encounter." In Zubiri's own words: "Es menester que después de haber aprehendido los objetos bajo quienes late, un nuevo acto mental reobre sobre el anterior para colocar el objeto en una nueva dimensión que haga, no transparente, sino visible, esa otra dimensión suya. El acto con que se hace patente el objeto de la filosofía no es una aprehensión, ni una intuición, sino una reflexión."
The object of philosophy, consequently, is transcendental. The sciences consider things such as they are; philosophy, by contrast, considers things as they are. Suchness and transcendentality-these are the dynamic polarities of philosophy in Zubiri's system; the structure which emerges from their interplay-essence-will be explored in Sobre la esencia. Philosophy is not alien to things in their suchness, such as they are; this seems to Zubiri to have been the error of the idealists (on the Platonic, and not the Hegelian, model, it is needless to say). Neither does it raise the qualitative properties of things to a transcendental status; this was the complimentary error of the empiricists. The task of philosophy is more difficult than these two positions assumed: it must isolate and characterize the transcendental function of things such as they are. This will become the theme of Sobre la esencia: essence: the structure of things as they are, such as they are, which establishes them as real. The pretension to do this may be a scandal to the scientists; but it defines the problem of philosophy in its central focus. The object of philosophy is not formally our mode of knowing; even less is it a kind of macro-object (the Idea) which emerges from the dialectical play of things. Neither is the object of philosophy the "Being" (ser) of things, in the sense given this concept either by Heidegger or by Hegel. Being-ser-does not (although Zubiri was inclined to think so in his Heideggerian period) fulfill the condition of ultimacy - the condition of transcendentality. As Zubiri wrote in an early (1931)  conference on Hegel: "el primer problema de filosofía, el filtimo ... de sus problemas no es la pregunta griega ¿qué es el ser? sino algo que está más allá del ser."
The comparison with Heidegger throws considerable light on Zubiri's position because the thought of Heidegger is a constant point of reference of his own. The effort has been made to establish a positive relationship between what Heidegger means by Being (ser) and what Zubiri means by "realidad." But this relationship proves under careful analysis to be untenable. Not that no relationship exists for, in fact, Zubiri develops the notion of "realidad" with a certain reference precisely to Heidegger's Being. Eventually, however, he decides that what Heidegger understands by Being is not sufficiently radical and ultimate to constitute an adequate response to the question of metaphysics. On the contrary, Being and reality appear to Zubiri to constitute two different formalities at the metaphysical level-formalities which must be carefully distinguished. Zubiri does not mean that his idea of "realidad" is to dislodge or substitute for Heidegger's notion of Being, but rather that the latter must be purified and transcended, but not negated, in. the idea of "realidad".
The Doctrine of Sobre la esencia
Between the earlier works of Zubiri such as Naturaleza Historia Dios and Cinco Lecciones and Sobre la esencia there is, as Ellacuria has remarked, not rupture but evolution. The path of this evolution is from concern with the idea of philosophy, seem principally from the point of view of the philosopher, to the construction of philosophy itself as the quest for "el ú1timo de las cosas"-from idea or paradigm of philosophy, that is, to actualization. Obviously the achievement of Sobre la esencia rests heavily on these earlier reflections for in them Zubiri had formed in dialogue with other philosophers, contemporary and past-according to his own dictum: "la historia de la filosofía no es cultura ni erudición filosófica. Es encontrarse con las demás filósofos en las cosas sobre que se filosofa"  -his own idea of philosophy of which the doctrine of Sobre la esencia is the actualization. As Ellacuria notes: "en su última etapa Zubiri ha dejado de lado el tema de que es la filosofía para ponerse de lleno a hacerla."
Per tÅj oÜsaj ½ qewra
It is this phrase from the Metaphysics of Aristotle that Zubiri chooses as the motto of Sobre la esencia and the choice is portentous: for his quest and that of Aristotle are identical though pursued in ways diverse and to significantly diverse terms. What Aristotle meant by oÜsa -not what he ultimately offered as oÜsa, that is, what in fact, in his view, fulfilled the condition of oÜsa, but what oÜsa is-what Aristotle meant by oÜsa, is that which is really real; that which is real in fact, to be sure (but this would be merely the object of science as Zubiri has already indicated) but the principle, that by which that which is really real is real. And this is clearly indicated by Zubiri as the end of all philosophic quest which he accepts as his own: "el ú1timo de las cosas"; not "las cosas," (though these make the only possible starting point and the eventual point of reference of all qewra), but "el último de las cosas." Ultimate from the point of view of philosophy as man's quest but principle, that is beginning, in an absolute sense from the point of view of what is really real: beginning and necessary ground of necessity and actuality.
At the very beginning a point of confusion threatens which Aristotle and Zubiri are equally concerned to avoid-the latter more concerned, perhaps, because he has seen this confusion posited and compounded by some of the greatest philosophers of the west: Plato, Hegel, in a word all those to whom the epithet "idealist," with whatever justice, is assigned. This confusion involves the notion of principle itself and resides in the substitution of the idea as principle for the real principle, or the principle of the real as itself supremely real - and the attempt to derive the latter from the former: the really real from the idea-and to demand that the latter conform to the former, compounding the problem by confusing the sollen and  the sein. But this, from the point of view of Zubiri, is an inversion of both the order of truth and that of reality. For clearly, he holds the truth is determined by what is. Truth is in the order of physis when physis is understood, as Aristotle understood it, as the actually real principle of what is actually real. For Zubiri, as Ellacuria writes, "una metafisica que no fuera suficientemente 'fisica' dejará de ser lo que es para convertirse en 1ógica, fenomenología, etc. " echoing the even more emphatic statement of Zubiri himself "La metafísica jamás podrá ser una 1ógica. Es insostenible el supuesto primordial de la metafísica hegeliana." What then is the "physical"?
Here Aristotle contributes on his part to the confusion from which this concept of the "physical" must be delivered. He makes the distinction fall heavily on the difference between natural and technical or artificial. Zubiri points out, however, that modern experience has shown that it is technically possible to make something that is perfectly real, real in the same sense that the real things are real. "Physical" must be taken as meaning, not that which has a "nature"; it must be recognized as the synonym of "real." Others have equated the "real" with the existent, the existential, the vital, the ontic, the ontological, the logical, the experimentable, etc. For Zubiri, none of these proves sufficiently "ultimate" to meet the conditions of the end of the metaphysical quest. The formula real-physical may be a pleonasm; if so, Zubiri replies "un puro pleonasmo pero muy útil." No empiricism is implied, Zubiri contends, in a false opposition, also traceable, at least in part, to Aristotle, between "physical" and "metaphysical." The physical has two aspects: one positive, the other "meta" physical (talidad-transcendental). That is to say, the "physical" must be understood as that which is real (i.e., as a matter of "fact"); this is the positive dimension of the physical. The "physical" must also be considered as the formal and ultimate principle of the real as such. These two aspects of the physical are not identical, but are related in a special manner which Zubiri will investigate extensively when he takes up the question of "transcendental function" in general and of the "transcendental function" of "essence" in particular. "Realidad física es realidad qua realidad, por tanto su carácter es eo ipso  un carácter metafisico." Metaphysical does not mean escaping from the physical into the conceptive order by way of predicative logic or any other mental construct, but means fixing the transcendental bond between what is actually real and the principle of its reality, not as such reality merely, but as real qua real. "Positive" knowledge focuses ,on what is real, i.e., what exercises reality, exemplifies, etc.; metaphysics addresses in what is real that which is its reality, that which establishes it as real, and not merely as actual or as this (such). This second enquiry goes beyond-"más allá"-the range of positive knowledge but the real in this second sense is entirely actualized in what is real in the first and hence can be sought successfully only when that which is real, in the sense of actual and such is taken as point of departure. The "más allá" does not imply a latent "other"; it is a depth dimension of what is actually real. Physical knowledge may, therefore, be schematized as "physical-positive" and "physical-metaphysical"; the former addresses what the thing is, its "talidad," in its "talidad," in its real concretion; the latter addresses the same thing insofar as it is real, not this or that real, but its reality qua real. Thus Zubiri transcends the dualisms: empirical-rational, ontico-ontological, etc., which have bedeviled Western metaphysics. He addresses the reality in its dual aspects as suchness (talidad) and as transcendental function and in the transcendental linkage between these. His philosophy is neither a "conceptive" or a "materialistic" nor yet a "physicalistic" metaphysics; neither is it a theory of objects (Meinong) nor an "ontology." It is a "realistic" metaphysics in the full sense of the word-for it goes directly to reality, to the reality which surrounds us, under its dual aspect. In his philosophy there is no opposition between scientific and metaphysical but a strict transcendental interfunctionalism.
These considerations lead ineluctably to the conclusion that metaphysics must be an intramundane inquiry. It takes its point of departure in the things that are real about us (mundo) to determine their dual aspect: what they are as real things (talitativamente) and in what their reality as real, and not merely as "talitative," resides. He writes: "Puedo proponerme ... describer la estructura y la condición rnetafísica de las realidades en cuanto tal." In his system, it is  true, this inquiry will not prove metaphysically ultimate; he will discover that the reality of the real things of the world is not itself ultimate: "se radicalizará últimamente, por intrínseca y rigurosa necesidad, la estructura metafisica de lo real en tanto creado." This is a further step, however, to which he is moved precisely by the initial inquiry as to the character of the real things of the real world as real. Such further inquiry "caería en el vacío si no se apoyara en una filosofía primera intramundana."
As a consequence, Zubiri rejects two of the pillars of classical metaphysics: analogy and the concept of general metaphysics. The latter would prescind from intramundane reality to address itself only to transmundane reality and its science would constitute "first philosophy," its object "ens ut sic." That "general metaphysics" in Zubiri's view must prove a phantasmagoria, a product of the speculative imagination. The question of what reality is can be met only from the ground of real things as they are in the world and as they are there present to the intelligence of man: sentient intelligence. Since reality presents itself to us only in real things and since the real things which are present to us as real are present to us as sentient intelligences and the things so present as real are the things of this world, there is no other point of departure for first philosophy, no other form first philosophy can take save that of an intramundane metaphysics. For this reason, the method of analogy, on which that general metaphysics rests, appears to him to be inadequate, for it supposes that the ways of being real may be many, when they cannot (although the ways of being [ser, estar siendo] may be, or that the ways of knowing the real may be many when they cannot). This intramundane metaphysics must therefore be accounted first philosophy, for only by way of this intramundane metaphysics can reality as principle be reached, even when this principle stands revealed as "más allá" the real things of the world, and of the world itself as "creado." For that "más allá" is not a phantasm beyond real things, but an inner depth of their reality. 
Intelligence and reality
One of the first questions to which Zubiri addresses himself is that of man's primary and fundamental access to reality. He replies to this question by his theory of "sentient intelligence." The theory of "sentient intelligence" as a theory of man's primary access to reality must be distinguished from the "epistemological question" or the theory of knowledge. The theory of the intelligence is logically antecedent to the epistemological question and every epistemological theory eventually reveals that it presupposes a theory of the intelligence in its account of what and how man can know.
For Zubiri the nexus between intelligence and reality is strict. What is understood by reality inevitably depends on the manner in which things present themselves to us, the way in which we encounter them. The determination of the fundamental way in which things present themselves to intelligence constitutes in itself a determination of reality. Zubiri avoids two extremes, which in the past have perplexed this issue: first, that of considering human reality, the reality of the intelligence itself, as closed, in such wise that it must go "outside" itself in quest of things and second that of considering it as pure openness so that reality becomes nothing but its precipitate. Rather, there is a mutual implication between them, but not necessarily one which implies a perfect equivalence of status in this reciprocity.
The salient feature of this mutual, but not equivalent, implication between intelligence and reality for Zubiri is the status of reality as principle with reference to the intelligence: "la principialidad de la realidad sobre la inteligencia." This status as principle means priority; the priority of the real means that the real is present to us as something the presence of which is consequent and subsequent to the real, as already real in its own character. This priority is not merely that of reality before the Ego, nor before reason, the predicative logos, the concept, intentionality or "unveiling." It is these and more. The character of this priority is based on the primary and  formal activity of the intelligence: actualization. This actualization, which must be called the very essence of intelligence, means that reality which is actualized of itself antecedently to its presence to the intelligence of itself compels the intelligence to actualize itself conformably to that antecedent reality. The intellective actualization takes place in the intellect and is of the intellect but the act which actualizes itself in the intellect is the act and actualization of the thing. Therefore what reality is (as distinct from merely what is real) appears formally only in the pure actualization of the intelligence, while what intelligence is appears formally only in the pure formality of reality as prius to intelligence. Together these moments yield-in due hierarchical relation-the real truth (verdad real).
In the case of the human intelligence, the force with which reality presents itself exhibits special characteristics. Human intelligence is not pure intelligence. Human sensibility renders it a sentient intelligence. Human sensibility renders reality present in impression; nevertheless it presents reality in its specific character as real. Man is "animal de realidades." His impressions are not mere "affections" of his own state but of reality itself. In this he differs from all other animals. Nevertheless, he has access to reality only by way of sensible reality given in impression; but that of which he has the impression is reality itself as it is "de suyo." This radical realism of the human intelligent condition is the basic principle of Zubiri's intramundane metaphysics.
Reality as principle
The principle, then, which permits us to explain the ultimacy and the totality of the real is, for Zubiri, reality itself in its character as "de suyo": its absolute priority in itself to the intelligence. The character of reality as principle is here anterior to and more fundamental than the principle proposed in other metaphysical systems. These others are, rather, derived from it-from the presence of the thing in its character "de suyo." This is true with respect to nature, to existence and the existent, to aptitude for existence, to essence as  classically understood and to Being (ser) in any and all of the three meanings assigned to it classically: copulative, substantive, transcendental. It is reality as "de suyo," the irreducible and non-deducible prius of all presence which is the principle by which all is totally and ultimately explicated.
Essence as the Structural Principle of Reality
Zubiri has insisted throughout his investigation that reality is to be reached only in and through real things, things that are real-that reality cannot be reached or attained by any logical process or dialectical ploy. In this sense it can therefore be said that reality itself has its principle in real things-"realidad está principiada en las cosas reales." What then is the reality of things that are real? His answer is-essence. Essence is the principle of the reality of real things, of things which are real. The real thing is its essence. And the essence itself, in its turn, must be understood both as quidditative and transcendental: as endowing the real thing with its quidditative or talitative status as a real thing of this or that kind and as real simpliciter.
Zubiri's concept of essence differs from all other concepts of essence heretofore advanced in the history of philosophy. For him, essence is identified and characterized through the function which it performs in the real thing, the thing that is real-"según la función que desempeña en la cosa real." When we answer the question: what function does the essence perform in the real thing, we indicate what the essence is as principle. Now the essence, in Zubiri's view, functions in three ways, does three things. It establishes the real thing in its quidditative and qualitative status as a thing of this or that kind: the status we indicate in replying to the questions what is it, what kind is it? Secondly, the essence establishes the thing, in its quidditative status, as real, real in the same sense in which all things, though quidditatively distinct among themselves or from one another, are real in the same sense. Finally it establishes the relationship  between these two orders: a relationship which Zubiri calls functional identity. The consequence of this functional identity is that in discourse of one order, the talitative, we speak of the reality of that order (and not of mere appearance, etc.) and in discourse of the order of reality we speak of all things that are real. But the ability to speak in this way is not constitutive; it is derivative; it reflects the complex structure of the real in its immanent quidditative and transcendental dimensions. The emphasis on the mutual immanence is crucial. For the transcendental is not then itself the transcendent (though the transcendent is by no means excluded from reality, may, indeed, on the contrary, be rigorously implied in it) but that which establishes the transcendental reality of all, each and every thing that is real. Neither is the talitative order something which accrues "accidentally" to a constant, self-identical substratum: the real in its transcendental character is realized and actualized only in the things that are real in their total talitative character.
This complex structure is what constitutes the world. The world is constituted by the fact that the order of suchness and the order of transcendentality are intrinsic to each other. The link between them consists in the fact that the "talitative" order performs a transcendental function. It establishes the real as "de suyo." Reciprocally, when the essence is considered transcendentally it is seen to establish the entire real thing as "de suyo" as "res" or "thing": the transcendental-talitative complex in its "de suyo" character.
Open and Closed Essence: the Person
There are two fundamental types of essence, Zubiri holds, the closed and the open. The distinction between them is highly refined but important since it is the basis for the concept of person. The closed essence is closed because, due to its "suchness," its "talitative" status, its entire transcendental reality is exhausted in being "de suyo" in itself. The open essence, by contrast, is open because it is not merely such as it is, but in addition, because of what it is (ser talitativo), it belongs to itself. It is formally and reduplicatively "suyo"; it possesses itself in its formal character as reality. The manner of being "suyo" is what constitutes a person.
 The structure of the closed essence is the reason why something is a fact: "hecho"; a closed essence, as structural principle, is a merely "natural". thing. The structure of the open essence is the reason why something is an event: "suceso." The open essence, as structural principle, is not merely a "natural" thing, though it is that too, but, even more, is an event-reality.
The "essential" then as principle is a structural principle. The essence is not only principle of the substantivity, but is the structural principle of all that the real thing is and of all its ways of being real. The essence, in its turn, is so real of itself-de por sí-that everything that is real is real only in function of it.