In his groundbreaking Enquiry Concerning Human Understanding IV,1 (1748) David Hume claims that „the observation of human blindness and weakness is the result of all philosophy, and meets us at every turn, in spite of our endeavours to elude or avoid it.” Hume’s skeptical approach to causation in particular and his ensuing critique of metaphysics in general is later met by Kant’s justification of metaphysics in his Critique of Pure Reason (1781). This essay aims to answer the question why Hume thought that we cannot have any experience of causation by first analyzing Hume’s thesis on the nature of human understanding, of substance and causation, secondly by stating the problem of causation and finally by extrapolating how Hume’s skeptical approach to causation is in fact a rejection of the rationalist world view in philosophy, i.e. an attack on rationalist metaphysics, and how his attack of the principle of sufficient reason roused Kant’s transcendental philosophy (Critique A 784 /B 812).
What makes Hume’s philosophy this outstanding? It is his vigour in bringing human understanding and nature to the fore of his reflections. Every science has a relation to human nature that cannot be avoided according to Hume as „they [all the sciences] are judged of by their powers and faculties”1. Thus, man himself cannot escape the boundaries of his own cognitive faculties. To avoid errors in his own philosophy, Hume states that the only reasonable foundation for science is experience and observation and that the origin of our perception lies within our “impressions”2. This begs the question what do we really experience? How do we know if substances exist? Hume disagrees with Aristotle’s (Metaphysics VII, 1028a10-14) definition of substance in terms of matter and form as well as substance and accidents by opining that there is no such thing as a pure substance. What we experience is a bundle of accidents (its qualities) but not the individual substance itself. Association among ideas (as copies of impressions) arises due to our experience of multiple cases. A new impression is formed in the mind out of a feeling of expectation by habit.3 Thus, our notion of substance is not objective in Hume’s way of thinking, but subjective. The source of necessity is not in the world, but in us as we project a regularity onto the world.4 The same holds true for another important notion of metaphysics: causation. Hume claims that we infer nothing from one single instance, but we do infer a causal link from multiple, repeated instances.
The problem of causation is in fact a problem of induction as neither the cause nor the effect and thus a necessary connection is given to us by our perception. In the strict sense, we cannot have any experience of causation according to Hume. Hume demonstrates his thesis by the example of the billiard balls. What do we see when watching billiard? There is billiard ball (b1) and billiard ball (b2) lying still on the table. Now one player (P) strikes b1 with his cue stick and b1 moves towards b2 which is then moving. What we see is two balls colliding and then moving at different times:
b1 moves at time (t1) and b2 moves at time (t2) → succession of movements
Thus, so far Hume is right: we only see a succession of movements, nothing more. What we don’t see is what really happens at the point of contact when the two balls are colliding. We don’t see any kinetic energy being transferred from b1 to b2. There is no direct observation at the exact point of contact. However, we observe a change after the collision:
b1 is coming to a stop (now stationary); but b2 is moving (in the same direction as b1 before).
According to the physics of collision, there is a change in momentum over time (impulse) in the balls. The velocity of b1 and b2, as well as the friction and direction of each ball can be measured (transfer of energy). The human eye however is at a loss to see all these details in this one second of the collision.5 We cannot observe and experience causal connections but we infer them nevertheless out of habit (b1 ⊃ b2). Causation is then some kind of regularity by habit that gives us the ideas of causation and necessity (Treatise 1.3.14.20). The problem of causation is a problem of induction as our premises may hold true (b1 moves and touches b2 which is then moving) but the conclusion (that b1 is the cause of b2) doesn’t follow necessarily. The problem with induction is the generalization from the observed to the unobserved. The generalized conclusion from induction cannot achieve absolute certainty due to the fact that in this case it is based on the assumption of nature as one homogenous nature that we would then be able to describe according to intrinsic laws of nature. In nuce, no true deduction a priori is possible (Enquiry 1. 23) and no true induction a posteriori is possible when it comes to causation as we cannot have any experience of causation. There is no objective necessary causality only subjective regularity by habit. Claims about causality are “matter of fact” and thus contingent, synthetic and a posteriori, but not necessary.6
Hume’s reflections negate any reality independent from a subject – thus independent from human understanding and its cognitive faculties. This claim is supported by Hume’s analysis of two central notions of metaphysics – substance and causation – which are criticized as mere bundles of associations that have their source not in the world but in us. In this way, metaphysics is not the highest form of intellectual pursuit7 of which humans are capable, but unfortunately just mere illusion on our part. This is shown when it comes to causation due to the fact that causation and its necessity is only an idea in us which we project onto the world. For Hume causation is an illusion and self-deception on our part as humans. In his Treatise, Hume writes that „men are mightily governed by the imagination”8 which explains the reason why we form a belief about causation on the basis of speculative imagination. Cause and effect are distinct and not to be discovered by reason (Enquiry I, 25). In this way, Hume challenges the Principle of Sufficient Reason that states that everything must have a reason, cause or ground when he denies any necessary relation (Treatise I, 3, 3). His attack of the principle of sufficient reason roused Kant’s transcendental philosophy: „ ... the remembrance of David Hume was the very thing that … first interrupted my dogmatic slumber and gave a completely different direction to my researches in the field of speculative philosophy.”9 Kant holds it with Hume that a „rebirth of metaphysics” (Prol. 4, 257) is approaching but he sets out on a different path than Hume by identifying not reason but imagination „impregnated by experience” as the culprit (4, 258) and by declaring Hume’s conclusion as wrong10. Kant’s investigation aims for a kind of critical metaphysics that looks for synthetic judgements a priori and culminates in his own critique of critical and pure reason (Prol. 4, 261). Kant sets out to establish a new method of metaphysics to negotiate between the empiricists (Locke, Berkeley and Hume) and the rationalists (Descartes, Spinoza, Leibniz) and to put it „on the sure path of a science” (Critique B vii). Causation is a category to order sensory impressions for Kant. His revolutionary core idea is „to explain the properties of observed phenomena by postulating a kind of activity in the observer”11 (Copernican revolution of philosophy). Reason is thus the source of every knowledge and answers human’s natural predisposition to go beyond the physical, beyond what can be read off the experiences.
What Hume and Kant agree about are the boundaries of human reason. Hume holds that true causality is impossible and not to be discovered by reason whereas Kant opines for the possibility of causality within reason – and thus the very possibility of metaphysics itself.