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The Xavier Zubiri Review, Volume 1, 1998, pp. 57-66
The Formality of Reality: Xavier Zubiri’s Critique of Hume’s Analysis of Causality
Thomas B. Fowler
President, Xavier Zubiri Foundation of North America
Washington, DC USA
Causality has been a pivotal concept in the history of philosophy since the time of the Ancient Greeks. After David Hume, however, many have questioned whether there is (or can be) any metaphysical meaning of causality, or valid inferences based upon it. Xavier Zubiri (1898-1983) has rethought and reformulated the question of causality in light of its historical roles, well-known criticisms, and relevant contemporary knowledge. In doing so, he has achieved a unique perspective on the subject which should be of great interest to those concerned with causality and any of its applications.
II. Hume’s critique of causality
The figure of David Hume looms large in the philosophical tradition of English-speaking countries; and his two famous analyses, of human apprehension and of causality, were the most penetrating up to his time, and continue to have great influence. As the culmination of British empiricism, Hume’s work is especially significant because he realized the importance of analyzing human apprehension both as a step in the development of a comprehensive philosophy, and in connection with the problem of causality. This task Hume undertook in his Treatise of Human Nature, Book I. By Hume’s time, causality had shifted from being primarily about things in the world, to being the way of guaranteeing the veracity of our perception of it. As a result, any attack on the principle of causality would inevitably have grave repercussions for knowledge about the world, or pretensions to it. A fortiori, claims to metaphysical knowledge would be devastated. There are thus three important aspects of Hume’s critique of causality: (a) the nature of causality as constant conjunction of disparate ideas; (b) the skeptical conclusions about our knowledge of the world following from this; and (c) the anti-metaphysical import of his argumentation.
In the course of his analysis of human apprehension, Hume, being a good empiricist, makes two claims that he believes to be self-evident: (a) all we know are our ideas of things, formed by the mind on the basis of sensory impressions; and (b) all knowledge, which is perforce propositional, must be either about relations of ideas or matters of fact. Knowledge based on relations of ideas includes logic and mathematics. But knowledge of the world is entirely founded upon matters of fact. Since the ideas involved in matters of fact can be conceived as separable, for example, the idea of a hammer and the idea of glass breaking, whereas those of mathematics cannot, for example two plus two, and four, there is a fundamental difference in the knowledge we can have. In particular, with respect to causality, the idea of a cause, such as a hammer, can always be conceived as being present without another idea, such as that of glass breaking. Hence there is no necessary connection between these two, simply a constant conjunction observed repeatedly and thus known from experience:
....Thus we remember to have seen that species of object we call flame, and to have felt that species of sensation we call heat. We likewise call to mind  their constant conjunction in all past instances. Without any further ceremony, we call the one cause, and the other effect, and infer the existence of the one from that of the other.
Now, for Hume, knowledge of the "external world" cannot come directly from the senses; they merely deliver impressions to the mind. A basic assumption Hume makes, never explicitly stated but always lurking just beneath the surface, is that all reasoning and understanding of the external world comes from the mind working on the content of these sensible impressions, be they pains, pleasures, colors, or sounds. The burden of inferring the existence of things outside of the mind then must fall upon the mind and those processes available to it, because what the senses deliver is inadequate to the task:
That our senses offer not their impressions as the images of something distinct, or independent, and external, is evident; because they convey to us nothing but a single perception, and never give us the least intimation of anything beyond.
Not surprisingly, because we can only know our ideas of things, and not the things themselves, we must rely on chains of causal reasoning to connect us to the "external world". Hume seeks to establish this in Part IV of the Treatise, where he argues that belief in the independent existence of things in the world must either come from the senses, reason, or what he terms ‘imagination’; and he dismisses the first two, leaving only the last, where he attributes the belief to coherence and constancy of impressions. This is nothing but constant conjunction, so in the end, it is causality which must carry the day: "...the relation of cause and effect is requisite to persuade us of any real existence ..." And perhaps more explicitly: "…any conclusion (about matters of fact) beyond the impressions of our senses can be founded only on the connection of cause and effect.".
For Hume, insofar as true knowledge of causes is possible, we can have apodiectic or metaphysical knowledge; but insofar as our knowledge of causes resolves into constant conjunctions, "true" knowledge about the "external" world, and a fortiori metaphysical knowledge, is impossible, leading to his famous pronouncement:
If we take in our hand any volume; of divinity or school metaphysics, for instance; let us ask, Does it contain any abstract reasoning concerning quantity or number? No. Does it contain any experimental reasoning concerning matters of fact and existence? No. Commit it then to the flames: for it can contain nothing but sophistry and illusion.
At the risk of oversimplifying, we may summarize Hume’s argument in the following schema:
(i) There is an absolute distinction between relations of ideas and matters of fact [empirical knowledge]
(ii) All we know are our ideas.
(iii) In the case of matters of fact, these ideas can be conceived separably.
(iv) Therefore we do not perceive any real effect of one thing on another, i.e., no causality in the classical sense.
(v) Therefore what we term ‘causality’ can be nothing other than mere constant conjunction of the idea of the cause with that of the effect.
(vi) We must use this to reason about how things are in the world, i.e., put us in touch with the world.
(vii) Therefore our knowledge of things in the "external world", and our knowledge of the external world itself, is problematic.
(viii) Constant conjunction is inadequate for metaphysical purposes; the stronger version of causality required.
(ix) Therefore metaphysics impossible. 
Given Hume’s point of departure, and his fundamental assumptions about the nature of human apprehension, human intelligence, and reality, Zubiri believes that there is no escape from his skeptical conclusions. Accordingly, Zubiri does not criticize Hume’s reasoning; rather, he analyzes the extremely deep-seated assumptions in Hume’s philosophy, to show that they are at fault and responsible for his erroneous conclusions.
III. The "classical" notion of causality attacked by Hume
What was Hume’s target? What notion or theory did he wish to demolish? Why was it important for him to do so? Causality as a notion had developed from the pre-Socratics through the Greeks, culminating in Aristotle’s analysis of it into four types. Further work was done by the Islamic philosophers, and later by the Medieval philosophers, though few substantive changes were made by them to the basic idea. The classical notion of causality and its principle characteristics, used by virtually all philosophers until Hume’s time, is summarized in Figure 1. Even Hume assumed that this notion of causality was monolithic, and sought not to refine or modify it, but to discard it altogether and replace it with a totally different notion.
IV. Zubiri’s Critique of Hume
Zubiri, with his accustomed thoroughness, has analyzed Hume’s philosophy in order to extract what important insights lie therein. Zubiri agrees with Hume and the other British empiricists that prior to development of any epistemology, it is first necessary to go one level deeper in order to fully analyze human intelligence. Such analysis is not a new theory, but is (or should be) only a pure and rigorous description of that intelligence. He also agrees that this analysis must be the basis of any theory of causality, and in particular, Zubiri agrees with Hume that "causes", in some metaphysical sense, are not given in experience. Finally, he agrees that there are significant problems with the "classical" notion of causality, which is in need of a complete overhaul. Nonetheless, he believes that Hume’s analysis of causality is fatally flawed in respect to all three aspects of it given above. The thrust of Zubiri’s critique centers upon five major areas:
- Hume’s analysis of human intelligence is radically false, rendering his arguments for causality as constant conjunction, for skepticism, and against metaphysics invalid.
- Hume’s notion of causality is inadequate for purposes he intended
- The locus of causality is formality, not content of impressions
- Functionality is correct replacement for causality, it is directly perceived, and therefore so is causality
- The "classical" notion of causality is valid only in moral sphere
Each of these will be discussed below. The mapping of Hume’s analysis to Zubiri’s critique is shown in Table 1.
a) Hume’s analysis of human intelligence is radically false
For Zubiri, Hume’s analysis of intelligence fails for four principal reasons: (1) human intelligence is sentient, not sensible; (2) not all types of knowledge of the world can be collapsed to a single paradigm; (3) no clear distinction exists between truths of reason and truths of fact; (4) truth is not solely or even primarily propositional. Understanding of all of these points is necessary in order to unravel the problems of Hume’s analysis
(1) Our intelligence is sentient, not sensible. Hume accepted the traditional notion that our intelligence is sensible, based on opposition between sensing and intelligence. The senses deliver some content to the mind, which then must figure out or reconstruct the "external world". One of the quotations from Hume given above bears repetition in this context: "…our senses offer not their impressions as the images of something distinct, or independent, and external…because they convey to us nothing but a single perception, and never give us the least intimation of anything beyond. [last italics added]
For Zubiri, this is radically false: the "impressions" we have are not sensory impressions but sentient impressions, that is, impressions of reality. These impressions have two aspects, content (what the impression is of) and formality (its mode of being delivered to us, reality, as something "in its own right", de suyo), both of which are indispensable in the knowing process. From this two important conclusions follow: First, it is not true that all we know are our ideas of things, understood as the content of impressions and our reflection upon them; we know much more: something about reality, albeit at the level of primordial apprehension, through formality. Reality is formality, and therefore Hume’s assertion that impressions "never give us the least intimation of anything beyond" is completely false. Their content does not; their formality does. Formality does not deliver the world to us intact; but it does begin the process in a modest but secure and indispensable way. Hume’s approach to the analysis of intelligence and the results it must yield about secure knowledge of realty was "all or nothing", and as he could not get the "all", he concluded the "nothing". A much more measured and subtle approach is in fact necessary, as discussed in (2) below.
Second, the empiricist notion of separable "ideas" floating around in our minds and, more or less by definition, connected with reality only indirectly, if at all, through some complex sensing mechanism, is thus completely inadequate as a description of the mental life. So Hume’s paradigm of knowledge, based on this notion of "ideas" and impressions, and hence his argument intended to establish causality as constant conjunction, founded on it, are both flawed at their root. Logically speaking, however, does not necessarily mean that his conclusions about the nature of causality is false. To establish that, note that we do not need causality and causal inference to reach reality—thus making our access to it problematic—as Hume erroneously concluded; because of the way we are constituted, we are directly in reality. Thus the skeptical and anti-metaphysical conclusions Hume draws are vitiated at the outset. Insofar as these are implied by Hume’s theory about the nature of causality, by modus tollens it is falsified. Therefore Hume’s assumption (i) is incorrect.
(2) Not all types of knowledge of the world can be collapsed into a single paradigm. Hume has made all types of knowledge of matters of fact—and thus of the external world—depend upon or be based upon causality, in turn identified with constant conjunction. This is operation of the intelligence at a very high level, after it has recognized objects, classified them, and begun to investigate their natures. It is asking for too much, too soon. The correct analysis must center on the far more complex and subtle three stages by which our knowledge unfolds: primordial apprehension of reality, logos, and reason. Since he failed to distinguish the basic act of sentient intelligence—primordial apprehension—from subsequent acts—logos and reason—which involve the intelligence in more discursive ways, Hume failed to recognize that the type and veracity of knowledge obtained in these acts differ sharply. Causality is known to us first at the level of primordial apprehension, which Hume completely overlooked, where certainty is achieved, indeed delivered to us. Subsequent knowledge, at the level logos and reason, also utilizes causality. At these levels, mistakes are possible; but because Hume only considered knowledge at these levels, and indeed only at the level of reason, he based his argument solely on the processes at that level, thereby implicitly (and erroneously) applying them to all knowledge of causality and through cau-
CAUSALITY IN CLASSICAL PHILOSOPHY
sality.  Because knowledge at the level of reason is fallible, he concluded that all knowledge based on causality was similarly fallible, and hence the notion was useless for many of its intended purposes.
(3) There is no clear distinction between truths of reason and truths of fact. Hume’s analysis of causality depends critically on this distinction, because he needs to establish that there is no connection between truths of fact. Were such a connection to exist, it would imply the existence of some sort of non-empirical knowledge—presumably metaphysical knowledge—that would undercut the thrust of Hume’s entire philosophy. The oversimplified nature of Hume’s analysis was immediately recognized by Kant, who split his "relations of ideas" into the analytic a priori and the synthetic a priori. But Kant experienced difficulty with respect to his classification of judgements, and even put the statements of physics—including causality—into the synthetic a priori category. It has never been possible to find a suitable way to make the division Hume had in mind (of course, for Leibniz, all truths are analytic in God’s mind, though not for us). For Zubiri, this is a pseudo-problem, arising because the nature of mathematics and hence of mathematical truth was not known. For Zubiri, the objects of mathematics are real by postulation, and therefore they are also known in a sentient intellection. So even they have a certain empirical content, though it is not empirical in the sense of facts about the physical world. Moreover, the nature of truth in mathematics is not different than the nature of truth for other things; there are only truths of necessary realities and truths of contingent realities. Evidence, ultimately sentient, leads us to affirm these truths on the basis ultimately of a process of discovery. Mathematics, for Zubiri, is empirical, but retains its character of correctness and rigor. But, in Zubiri’s reality-based philosophy, "…the real thing determines the correctness of its intellection".
(4) Truth is not solely or even primarily propositional. For Hume, truth is a characteristic of propositions expressing relations of ideas or matters of fact. Truth of the former, he believes, can be known with certainty, but not the latter. But in fact truth is not identifiable with "judgements" in the classical or Kantian sense. Rather, there is a primary meaning of truth, real truth, which refers to the presence of an impression of reality in the mind, i.e., sentient impressions. This is important because, as will be seen below, causality arises from just such an impression: The truth of statements or propositions such as "all men are mortal"—the only place where Hume thought it exists—is derived or dual truth.
Hume’s Analysis of intelligence flawed
Locus of causality is formality
Adequacy of Hume’s notion of causality
Causality really is functionality
Moral theory and causality
Nature of causality as constant conjunction
skeptical conclusions about knowledge of the world
Table 1. Mapping of Hume’s Analysis of Causality to Zubiri’s Critique
b) Hume’s notion of causality is inadequate for the purposes he intended
Hume believed that science requires causality; and because science is undeniably both knowledge and empirical, he made every effort to ensure that his new explanation of causality could support the development and use of scientific knowledge. Hume (and the science of his age) inherited the notions of strict determinism and uniformity from classical causality, and made them part of the fabric of even his new notion of causality:
…the same course of reasoning will make us conclude, that there is but one kind of necessity, as there is but one kind of cause, and that the common distinction between moral and physical necessity is without any foundation in nature….As objects must be conjoined or not, and as the mind must either be determined or not to pass from one object to another, it is impossible to admit of any medium between chance and an absolute necessity… [italics added]
The inadequacy of Hume’s notion of constant conjunction as a definition of causality, both in science and elsewhere, has been known for some time, and widely criticized. Zubiri discusses it indirectly, in connection with different though related issues. For Zubiri, modern science has forced a complete rethinking of the notion of causality, and in particular, it has shown that causality cannot be characterized monolithically as deterministic. Rather, determinism is merely a type of causality:
It is possible that things are interrelated by determinate links, i.e., that the state of the electron in an instant of time determines its later course. But what Heisenberg’s principle affirms is that such a determinism has no physical meaning, on account of the impossibility of knowing exactly the initial state. If this impossibility were accidental, i.e., if it depended on the subtlety of our means of observation, Planck would be right. But if it is an absolute impossibility for physics…the presumed real determinism escapes physics. It no longer has physical meaning. In such case, the Uncertainty Principle would not necessarily be a renunciation of the idea of a cause, but rather of the idea that classical physics formed of causality…Causality is not synonymous with determinism; rather, determinism is a type of causality.
The implication of this is quite clear: insofar as Hume’s theory of the intelligence requires causality to be deterministic, it is erroneous, as is his theory of causality.
c) The locus of causality is formality, not content of impressions
At the level of primordial apprehension, Hume failed to distinguish content and formality of reality in impressions, and indeed he totally overlooked formality. But as Zubiri has demonstrated, every impression has both content and its packaging, formality. Hume assumed that impressions are exhausted by their content, which therefore must be the locus of causality—and hence of all of our knowledge of the external world. But in fact it is formality, not content, which delivers reality, including causality, to us, at this most important level, that of primordial apprehension. Zubiri notes:
 content of the impressions but their formality of reality.
For Hume, causality is not given, but only temporal succession. Now, I have just said myself that causality [in the classical sense] is not given. But Hume did not notice that there are two different aspects of the question. First of all, he did not see that temporal succession is just a form of functionality. In the second place, the succession is not the succession of two impressions, but the same impression of reality, one which is of successive nature—which means that what is essential about functionality does not concern the
Thus it follows that causality has nothing to do with constant conjunction of the content of disparate ideas (or impressions), but with the formality of reality of a single impression of reality, of successive nature.
In other words, an impression of successive events gives the functionality (causality) and the reality of the succession at the same time. Thus through formality, functionality does give us knowledge of reality, so that Hume’s skepticism is misplaced. Zubiri would agree that if we had to rely solely upon reasoning which utilizes the content of impressions as the basis for our knowledge of reality, we could not escape Hume’s conclusions; content alone cannot bear this burden. We do in fact rely on such reasoning for much of our knowledge, at the level of logos and reason; but (i) all such knowledge would be impossible if reality were not first delivered to us in primordial apprehension, and (ii) even at these levels, reality is still formality, the de suyo, and so things such as electrons and empires act on other things qua real, in virtue of being "in their own right".
d) Functionality is the correct replacement for or interpretation of causality
Zubiri observes that that on which we base our knowledge, whether at the level of primordial apprehension or at the higher levels, is not constant conjunction. What was the idea behind causality in classical philosophy? It was to express a particular type of relationship between two things (or events, or processes). This relationship has the characteristics discussed above in section III. This may or may not have been adequate in the time of Aristotle, and up through the medieval period. But we now know that things can be related in many more ways than can be adequately described by the deterministic paradigm of classical causality. To describe this situation, Zubiri borrows a term (and idea) from mathematics: that of functionality. Functionality, sometimes describable only in mathematical language, is a much more general way of describing relationships among things. These relationships may be among more than two things, and may involve statistical ideas. Functional relations may or may not involve causality in the traditional sense, or Hume’s version, constant conjunction; functionality is a much broader concept, capable of supporting inferences such as counterfactual conditionals which are beyond the range of constant conjunction. Furthermore, functional relationships may be—and indeed often are—statistically based, for which constant conjunction as an explanation is hopelessly inadequate. Functional relations exist for all three levels of intelligence, beginning with primordial apprehension.
The importance of functionality can be best understood through Hume’s own example of the ringing of a bell when its cord is pulled:
In Hume’s example, the ringing of the bell just follows upon the pulling of the cord. Now, it is not the case that the bell’s ringing is qua ringing a function of the pulling of a cord qua cord [these concepts operate at the level of logos]; rather, the fact is that it is the reality of the ringing qua real [i.e., its formality] which is a function of the reality of the pulling of the cord qua reality [i.e., its formality]. And this is something perfectly given, even supposing that the ringing were not a function of the pulling of the cord.
Or to paraphrase Zubiri’s discussion, the ringing of the bell is apprehended as real in a primordial apprehension, the same one in which the pulling of the cord is apprehended as real. This is functionality at the level of primordial apprehension, not at the level of logos or reason, where Hume was looking. Thus the ringing of the bell is apprehended as a real function of the pulling of the cord, whether or not the pulling of the cord actually operates the bell by itself. Moreover, it would still express a relationship even if pulling the cord only made the bell sound 60% of the time, though it could not be Hume’s causality:
Functionality is functionality of the real inasmuch as it is real. In this sense it is a concept which encompasses many possible types. This formality, this "by" as such, is given in the impression of reality. Hume’s whole critique is based upon the content of sensing, but he erred on the matter of formality.
Understanding of the functionality of the bell ringing operation through logos and reason, e.g., through the physics of motion of the bell and clapper, the nature of sound waves, their generation through vibrations of the metal bell, and so forth, is much more difficult. So it is not surprising that if one tried to based our knowledge of reality on the achievement of certainty there, skepticism would be the natural result.
e) The "classical" notion of causality is valid but only applicable in moral sphere
Hume attempted to ground ethics on sentiment, and utilize causality there as constant conjunction, just as in physics (refer to quotation in section (b) above). This attempt was never very convincing, but it does point to the need for causality to establish the morality of actions. For Zubiri, causality in the traditional sense of a real power exerted by one thing on another could never be established because the causal nexus is too complicated. Causality, in this classical sense, can only be applicable to things that are substances in the full sense; but strictly speaking, because of the openness of reality, only the universe as a whole fulfills this requirement. However, in the case of human beings, the situation is somewhat different because we are, in certain respects, substances in the required sense, and as a consequence we know ourselves in a way which is impossible for us to know the world. Zubiri discusses the matter in some detail in connection with Kant. He also notes that man, as an autonomous being, a "relative absolute", must make himself as a person with the opportunities and things at his disposal: "Man exercises a causality in the line of being able to be and having to be in a certain way with that which he really and truly is. And man’s making himself consists in this." Such knowledge, of course, is something which goes beyond merely empirical knowledge, and also establishes the existence of metaphysical knowledge which Hume thought to be impossible.
Hume was correct in his insistence on the need to develop a theory of the intelligence, and on his observation that we do not perceive causes sufficiently well to base metaphysical conclusions about the world on them. But Hume’s analysis of intelligence and of causality, and the skeptical conclusions he draws from it, are all defective because he does not recognize the three stages involved in knowledge of the world, and the unique characteristics of the first, primordial apprehension of reality. In particular, he does not realize (1) that at that level, functionality is associated with the formality of reality of impressions, not their content; and (2) that reality in the most fundamental sense is given to us directly—there is no need (or even possibility) of causal chains to reach it. Hume attempted to place the entire responsibility for connecting us to reality on the content of impressions, a burden they cannot bear. Causality, in the classical sense, is a special case of functionality. Causality in the traditional sense of real power to influence external things comes into play in the moral and personal sphere.
 David Hume, Treatise of Human Nature, ed. by D. G. C. Macnabb, Cleveland: World Publishing Company, 1962, Section II, p. 240.^
 Treatise, p. 240.^
 Treatise., p. 245ff.^
 Treatise., Section IX.^
 Treatise, p. 74.^
 David Hume, Inquiry Concerning Human Understanding, Section XII, Part III, paragraph 15.^
 Xavier Zubiri, Inteligencia y realidad, Madrid: Alianza Editorial/Sociedad de Estudios y Publicaciones, 1980, Prologo.^
 Xavier Zubiri, Inteligencia y logos, (Second volume of trilogy, Inteligencia sentiente), Madrid: Alianza Editorial/Fundación Xavier Zubiri, 1982, p. 39-40. (Hereafter, IL).^
 Xavier Zubiri, Naturaleza, Historia, Dios, 5th ed., p. 287-304 (Hereafter, NHD).^
 Ibid., p. 240.^
 Xavier Zubiri, Espacio, Tiempo, Materia, Madrid: Alianza Editorial/Fundación Xavier Zubiri, 1996, p. 479-480.^
 Díaz Muñoz, Guillerma, Zubiri y la matemática: un nuevo constructivismo, doctoral thesis, Universidad Autónoma de Madrid, 1994, ch. 5, sec. 2.2.^
 IL, p. 231.^
 Treatise, p. 171.^
 NHD, p.288, 291.^
 IL, p. 40.^
 IL, p. 41.^
 IL, p. 41.^
 Xavier Zubiri, Estructura dinámica de la realidad, Madrid: Alianza Editorial/Fundación Xavier Zubiri, 1989, p. 90.^
 Xavier Zubiri, Los problemas fundamentales de la metafísica occidental, Madrid: Alianza Editorial/Fundación Xavier Zubiri, 1994, p. 229.^
 Xavier Zubiri, Sobre el sentimiento y la volición, Madrid: Alianza Editorial/Fundación Xavier Zubiri, 1992, p. 112.^
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