into the nature of the cause or of the effect were correct, Hume argued, we should be able to see causal necessity from one instance. We could then say that it was impossible for the one object not to follow, or be conceived not to follow. But in fact, we can conceive of causes and their effects as unconnected, and what is even more important, we never conclude that there is a causal connection until we have seen the same events related in the same way a number of times. As there is nothing in several instances repeated that there is not in any one of them, it cannot be that anything within the objects gives rise to the idea of necessity.1 The cause must then lie in the mind, not in the objects:
Tho’ the several resembling instances which give rise to the idea of power have no influence on each other, and can never produce any new quality in the object … yet the observation of this resemblance produces a new impression in the mind, which is its real model. … Necessity, then, is the effect of this observation, and is nothing but an internal impression of the mind, or a determination to carry our thoughts from one object to another.2
The necessity assumed is not, and cannot be, a necessity of reason (i.e., intuitively or demonstratively understood); it is only a necessity of feeling, in short, a belief, and it arises from custom.
Hume thus reduced what had formerly been described as an intuition about the essence of things to nothing more exalted than an experience of a customary conjunction between two objects. Again, he was involved in the difficulty that afflicted his analysis of abstract ideas. He did not explain what enabled the mind to recognize “like” causes, or customary conjunctions. If, in fact, we can know only what we have already experienced, how can we depart from experience to recognize an object’s similarity to the one we have experienced in the past? But Hume’s attention was concentrated on what he was denying, that the causal inference was an operation of reason enabling man to discover the rational pattern imprinted on nature by the Divine Mind. Having shown that merely custom explained the feeling of necessary connection between cause and effect, Hume left “those who delight in the discovery and contemplation of ‘final causes,’ “ to “employ their wonder and admiration” elsewhere.1 He had also eliminated the supposed difference between “moral” and “physical” necessity, for he had shown that all necessity lay in the mind, and arose from the influence of the repeated, constant conjunction of two objects. Instead of necessity there was only chance, reduced to order by custom.2
He had demonstrated, Hume felt, that reasoning never gives rise to “a new original, simple idea,”3 that nowhere in man is there anything “like this creative power, by which it raises from nothing a new idea, and with a kind of Fiat, imitates the omnipotence of its Maker. ”.”4 It seemed that human thought possessed unbounded liberty and power, but closer examination revealed that it is “confined within very narrow limits,”
and that all this creative power of the mind amounts to no more than the faculty of compounding, transposing, augmenting, or diminishing the materials afforded us by the senses and experience. … In short, all the materials of thinking are derived either from an outward or inward sentiment; the mixture and composition of these belongs alone to the mind and will.5
The implications for the activity of philosophizing were not comfortable. By reducing reason to a combination of imagination and sense impressions, Hume had made it impossible to say why what we believe is true, or in any way to establish the certain validity of our beliefs. He had described the workings of the mind, but in doing so had merged logic with psychology; his description of the causal relationship had made it impossible to explain it. Most awkward of all, his method had not accounted for his own philosophizing. The picture he had drawn of the limitations of human knowledge ruled out his own ability to construct it. He had in fact disproved his tide to prove or disprove anything. His Treatise could only be a miracle, unrelated to the rest.
It had not, however, been an interest in discovering the nature of knowing, for its own sake, that had driven him into philosophy. The point of his argument was moral, rather than epistemological, and this he made perfectly plain by concluding his analysis of human understanding with a section on the reason of animals.
Having demonstrated how the human operation that had hitherto been traced to a spiritual power, reason, was no such thing, he then turned the question round. He showed man to be standing closer to the animals than to God, not because, as Hobbes said, man was brutish but because animals were no less spiritual than he. At times Hume almost paraphrased Montaigne to argue that insofar as men reasoned, animals did too. It had long been observed, he said, that animals behaved very much as men did, and that they adapted means to ends in the same manner. It was only natural to assume that the causes of similar behaviour were similar:
We are conscious that we ourselves in adapting means to ends are guided by reason1 and design, and that ’tis not ignorantly nor casually we perform those actions, which tend to self-preservation, to the obtaining pleasure and avoiding pain. When therefore we see other creatures, in millions of instances perform like actions, and direct them to like ends, all our principles of reason and probability carry us with an invincible force to believe the existence of a like cause. … The resemblance betwixt the actions of animals and those of men is so entire in this respect, that the very first action of the first animal we shall please to pitch on, will afford us an incontestable argument for the present doctrine.2
Hume pointed out that this was not just a parenthetical observation but at the heart of his argument—“This doctrine is as useful as it is obvious, and furnishes us with a kind of touchstone, by which we may try every system in this species of philosophy.”3 It is easy, he declared, to test any hypothesis advanced to explain a mental operation by asking whether it applies equally well to men and beasts. Other philosophical systems supposed “a subtilty and refinement of thought” in a degree that exceeded the capacities not only of animals but also of most human beings. His own system, however, could “equally account for the reasonings of beasts as well as for those of the human species.” And it was the only philosophical system that could. Since it is generally admitted that beasts do not perceive any real connection among objects, or form any general conclusions, yet learn from experience and adapt their behaviour accordingly, it is evident that “rational” behaviour does not depend on reason in the sense given it by philosophers: “I assert they [rational actions of animals] proceed from a reasoning, that is not in itself different, nor founded on different principles from that which appears in human nature.”1
Here perhaps Hume was more artful than candid. The force of his argument about animals is certainly that if we do not need to postulate a creative reason in order to explain the behaviour of animals, we need not do so for humans. But he avoided making an offensive statement by reversing the order, and declaring that, “All this was sufficiently evident with respect to man. But with respect to beasts there cannot be the least suspicion of mistake; which must be own’d to be a strong confirmation, or rather an invincible proof of my system.”2 Hume was tactful, but his meaning is clear. Was it not odd, he asked, that men took their own reason for granted, but were astonished by the instinct of animals? It was only because they had not been able to reduce animal instinct to the same principles. In the light of his new philosophy, however, the problem vanished, because it removed the apparent distinction between human reason and animal instinct. It discovered reason to be “nothing but a wonderful and unintelligible instinct in our souls which carries us along a certain train of ideas, and endows them with particular qualities, according to their particular situations and relations.” It was an instinct that resulted from habit, and “habit is nothing but one of the principles of nature and derives all its force from that origin.”3 Man, then, was not outside or beyond nature, but part of it, merely a more elaborate animal: “Everything is conducted by springs and principles, which are not peculiar to man, or any species of animals.”4 Montaigne’s essays had been given a metaphysical foundation.
That Hume had done something radical, all his contemporaries sensed. What the radicalism consisted in, however, they did not quite grasp. Only Kant recognized the full implications of Hume’s theory. He saw the import of Hume’s concentration on the “necessity” of the causal relation, that he had denied not the reality of causation