Kant’s Response to Hume in Prolegomena
I will first present Hume’s argument for the impossibility of causality. Then, I will explain how Kant’s proof of a priori judgments aims to overcome such Humean skepticism. Submitted for PHIL 2103 on December 12, 2020.
Metaphysics would be under much duress if causality was indeed, as Hume suspects, impossible to obtain. For without causality, there would be no extension of knowledge in metaphysics, which Kant describes as our “proper object in studying it” (273). In his Prolegomena to Any Future Metaphysics, Kant sets out to rescue metaphysics as a science by saving pure concepts of understanding, like that of causality. In this paper, I will first present Hume’s argument for the impossibility of causality. Then, I will put forth Kant’s efforts to overcome the Humean skepticism by proving the possibility of synthetic a priori judgments.
In his An Enquiry Concerning Human Understanding, British philosopher David Hume exhibits his skepticism towards causality as a quality that exists in the objects themselves. Following Hume’s proposition that “[a]ll our ideas are merely copies of our impressions,” which he believes should not warrant much dispute, for an idea to be meaningful, we must have felt it “previously through either our external or our internal senses” (30). When Hume attempts to find the source of the idea of causality, however, he finds himself unable to do so. If we look to external objects, Hume says, we cannot find any quality that makes one thing an “infallible consequence” of another (30). All that our outer senses perceive when we witness a billiard ball hit another, for instance, is that motion follows from the impact; we do not know whether the impact has caused the motion (30). Furthermore, Hume states that if our mind is truly capable of the idea of causality “through thought and reasoning”, we should have been able to “foresee the effect” when we externally experience something for the first time, but we know that this omniscience is not present in us (30-31). Therefore, external impressions cannot be where our idea of causality originates.
Hume then considers the possibility of internal impressions, or “our reflection on the operations of our own minds,” as the source of our idea of causality (31). On the surface, this theory appears to be true, for it seems that whenever I will my body to move, and suppose it does, I should be able to internally experience a “necessary connection” between my mind and my body (30). Yet upon closer examination, we realize that we never discover the “energy” through which the will commands a movement. All we know, like in the case of external impressions, is that the motion “follows the command of our will” (30). The reason we think that causality exists among events is due to “a feeling that accompanies the imagination’s habitual move from observing one event to expecting another of the kind that usually follows it” (37). But repeatedly observing a sequence of events still does not necessarily prove a connection between these events (37).
In summary, because not “a single instance of connection” has ever “appeared as an impression to our outward sense or inward feeling,” what we mistake for necessary connections, or causality, are in fact loose and separate sequences and associations (36). As such, Hume states that the idea of causality is meaningless both philosophically and in common life usage (36).
Before attempting to reclaim the idea of causality, and then metaphysics, German philosopher Immanuel Kant recognizes that he first needs to establish the possibility of synthetic a priori truths, whose existence Hume must have deemed impossible for him to conclude causality’s possibility. The source of metaphysical cognition, being beyond experience by definition, cannot be a posteriori judgments, which come from experience, and thus must be a priori judgments, which come from “pure understanding and pure reason” and carry with them necessity (265). Our judgments are further divided into analytic judgments, which adds nothing new to the content of cognition, and synthetic judgments, whose predicates add to one’s concept of the subject beyond what is already contained in the universal concept of the subject (266-267). All analytic judgments are a priori, whereas synthetic judgments can be either a posteriori or a priori (267).
The reason synthetic judgments can be a priori is less obvious, for it seems that only judgments of experience, which are a posteriori, can be synthetic. This assumption, Kant believes, is gravely mistaken (268). Kant argues that proper mathematical judgments are also synthetic, although they are a priori, as they are necessarily true, instead of a posteriori (268). That is, contrary to what many “who have analyzed human reason” concluded, Hume included, mathematical judgments are not analytic but synthetic. For instance, if we say the mathematical judgment 7 + 5 = 12 is analytic, we are concluding that the concept of 7 + 5 contains in itself the concept of 12 (268). But when we think of 7 + 5, we are only thinking of the concept of addition of two numbers, and the answer 12 was not contained in 7 + 5—after all, we do not immediately know the answer to all additions (268). Instead, intuition, in addition to the principle of contradiction, allows us to add numerical units successively and synthesize a result (268). Geometric principles are also similarly synthetic: the concept of the shortest path between two points, which is the predicate, is not contained in the concept of a straight line, which is the subject, because a straight line contains a quality, not a quantity that measures whether something is shorter than another (268). In both arithmetic and geometric judgments, intuition “makes the synthesis possible” (269).
Because mathematical judgments appear “apodeictic and necessary,” i.e., a priori, the intuition that synthesizes the concepts to form them must be “some pure intuition” as opposed to an “empirical intuition,” which is our a posteriori sense-perception that forms judgements of experience (281, 283). In Kant’s view, these pure forms of intuition are space and time, because geometry cannot exist without the pure intuition of space, and the concept of numbers in arithmetic is formed through addition of units in the pure intuition of time (283). This is not to say that space and time are things in themselves. Rather, Kant is proposing that space and time are “formal conditions of our sensibility” that are always present and precede our sense-perception (284). That is, our intuition is “characterized” by space and time before having been acquainted with them and before they are presented to us through some material object (284). This gives rise to the possibility of mathematical judgments as synthetic a priori truths, because we are examining them as they “appear to us” within our mental framework of the pure intuitions of space and time, not as they are in themselves (284-285).
Such is also the case for natural sciences, which strives to attain cognitions of the “existence of things … as it is determined according to universal laws,” or laws that necessarily exist outside of our experience (294). By definition, a posteriori cognitions that are based on our experience is incapable of doing so. Yet universal laws of nature like “substance is permanent” and “every event is determined by a cause according to constant laws” actually exist, they must be cognized a priori (295-296). As is the case in mathematics, cognizing a priori necessary conformity is possible because our inquiry of natural sciences extends “not to things in themselves, but to things as objects of possible experience”—that is, our inquiry only extends to objects as they appear to us within our pure intuitions of space and time (296). If we were to form cognitions of natural sciences based on objects as they exist in themselves, then we would be obliged to form hyperphysical concepts of the objects’ nature, whose actuality cannot be confirmed by experience and thus may be erroneous (296).
If Hume had considered the possibility of synthetic a priori truths by examining objects of mathematics and of natural sciences as they appear to us and not in themselves, Kant conjectures, Hume would have been able to arrive at the pure concept of causality, because claims about causality would then only be about claims as they appear to us, which we can ascertain (272). Hume would subsequently discover synthetic a priori mathematical judgments that are included in causality (272). And because Hume is such an “acute man,” he could not possibly subject necessary mathematical axioms to experience (272-273). Since the basis of mathematical judgments is not a posteriori, so long as they are regarded as representations in space and time, the necessity in them would not be “fictitious and a mere illusion produced in us by long habit” (311). Likewise, metaphysical judgments such as causality, which contains mathematical judgments, would not be based on “mere experience,” because an attack on causality would be an attack on mathematics, which Hume is too acute to do (273).
Kant then distinguishes between judgments of perception, which are “subjectively valid,” and judgments of experience, which are “objectively valid”(297-298). The former requires nothing more than our “logical connection of perception,” whereas the latter requires “special concepts originally generated in the understanding” in addition to our empirical intuition (298). With the help of such “pure a priori concept[s] of understanding,” we can turn judgments which merely contain subjective validity into those that are “universally valid, and hence necessary” (298). To do so, Kant provides the following schema: first, in reference to the logical table of judgments, recognize a judgment of perception in terms of its quantity, quality, relation, and modality; then, apply the concepts of understanding listed in the transcendental table to form judgments of experience (303). For example, if I apply the judgment of perception “my dog is pink,” which is logically assertoric, the judgment of experience I can form according to the concept of understanding is “my dog exists.” Because these pure concepts contain the “a priori conditions of all synthetic and necessary judgments,” we may possess the sorts of a priori principles of natural science shown in the pure physiological table (303, 305).
One of such a priori principles of natural science which has necessity is the principle of causality (295). Kant agrees with Hume that we cannot recognize a “necessity of connection, or concept of cause,” if we were to only think of causality as an object in itself that we can sense (312). But if we were to think of causality not as something empirical to be sensed in the world. That is, the concept of causality is inherent in our apparatus, it is always present in shaping our judgments of the objects we experience; we are not free to escape from it. Realizing the concept of causality as a pure a priori concept of our understanding, we can then establish the principle of causation (295, 312). Even so, it should be noted that Kant is in no point suggesting that we know anything, causality included, about objects as they exist in themselves, but only as they appear to us within our mental framework of space and time.