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to cause change, he pointed out, could not be resolved into either sensation or reflection; the mind added to the materials supplied by sensation and reflection an idea of its own creation, the idea of cause and effect. The idea of substance was also admitted by Locke, and made equally independent of sensation and reflection. “I never said that the general idea of substance comes in by sensation and reflection or that it is a simple idea of sensation or reflection,” he replied sharply to the criticism of the Bishop of Worcester, “for general ideas come not into the mind by sensation or reflection, but are the creatures or inventions of the understanding, as I think I have shown.”² For all his talk of the dependence of ideas on sensation, intuition was, after all, fundamental for Locke. It furnished all the basic principles of certainty and knowledge.³

      Locke’s moral theory, which Clarke took over, depended entirely on the assumption that man has a rational faculty linking him to God. Reason was declared to be “natural revelation where the eternal Father of light and fountain of all knowledge communicates to mankind that portion of truth which he had laid within the reach of their natural faculties.”⁴ For Locke, no less than for the Cambridge Platonists, reason was “the Candle of the Lord” in man; it alone could discover the law by which he was to govern himself. As this law was nothing other than the rule God had set down for the governance of man, the nature of God and of virtue were one. Thus morality without God was unthinkable. That was why, despite his desire to extend toleration, Locke excluded atheists: “The taking away of God, though but even in thought, dissolves all. …”¹ And for the same reasons, Locke counselled parents to teach their children how to make their irrational, sensual, animal natures submit to the rule of reason.

      Locke nevertheless performed a most useful service for Hume. By distinguishing the creative, intuitive, synthetic powers of the mind from the passive, discursive, analytic powers of reflection and demonstration, Locke defined Hume’s problem. “Creative” reason was what the ancients called “logos” or “nous,” that is, the faculty of apprehending necessary truths, of discovering the essence of things, the permanent causes or reasons for the existence of facts experienced by the senses. It enabled man to transcend both matter and mortality, and to deal with eternal things—first causes, human destiny, the relation between man and God. Reason was man’s participation in the supreme rational power that shaped the universe, that imposed order on the chaos of matter and directed the world towards a rational end. Whereas the senses, memory, and imagination, which man shared with animals, enabled him to perceive individual things and to remember and collect his impressions, his intellect apprehended the principle of these things. It gave him knowledge of general truth that was not merely the sum of particular truths but a genuine insight into the nature of things, into final and formal causes.

      In order to sever man from God, Hume saw that he needed only to deny “creative” reason. He could then redefine reason as a more limited power to receive ideas, to compare and analyse them, a power that was impressive and distinctive but did not carry man beyond nature. The result would be a “com pleat system of the sciences, built on a foundation almost entirely new, and the only one upon which they can stand with any security.”²

      Since in the terms of Locke’s analysis, the work of the creative intellect was to frame abstract ideas and discover relations, these operations absorbed Hume’s attention. If he could have traced them to some faculty other than intuitive reason, he would have destroyed the argument for placing man above the rest of nature and near to God.

      At the height of his philosophic enthusiasm, Hume was moved by an ambition to follow the method of Newton, “the greatest and rarest genius that ever rose for the ornament of the species,” who had admitted “no principles but such as were founded on experiment.”¹ He too would disdain any traffic with occult qualities and reduce all phenomena to the simplest causes without, however, probing too far. In this mood, he produced a largely mechanistic analysis of mental phenomena. He began by calling Locke’s ideas “perceptions,” and resolved them into two kinds, “impressions” and “ideas.” Impressions included all the “sensations, passions, and emotions as they make their first appearance in the soul”; ideas were the images of these perceptions in thinking and reasoning. Ideas and perceptions were analogous to physical atoms, simple and separable, but connected by association, “a kind of attraction which in the mental world will be found to have as many extraordinary effects as in the natural, and to shew itself in as many and as various forms.”² The rest of the Treatise was devoted to showing that every activity of the mind could be explained without recourse to reason in the traditional sense. To this end, Hume occupied himself primarily with two topics, “Space and Time” and “Causation.”

      The ideas of space and time had always been taken for a prime example of abstract ideas, for they included the notion of infinity and seemed more than any other idea to go beyond the natural capacities of the mind. The human mind was finite, and yet it seemed able to conceive of infinity; in many of its operations it depended on sensation, and yet it had an idea of a vacuum or space where there was nothing that could give rise to sensation. Thus the ideas of space and time had the “air of a paradox,” were “contrary to the first and most unprejudic’d notions of mankind,” and therefore, Hume explained, “greedily embrac’d by philosophers,” as showing the superiority of their science, “which coul’d discover their opinions so remote from vulgar conceptions.”³

      Causation was crucial for Hume because it was the only idea of relation that seemed to involve a truly intellectual element. All the others Hume showed to be merely a comparison of ideas or impressions received through sensation. With these mental operations, he had no quarrel, as they implied no creative power of the mind to add something of its own, but merely a power to receive and manipulate ideas. Hume took pains to point out that mathematics was a case of the latter, that there was nothing remarkable in the ability to think of a triangle. Mathematicians liked to pretend that the objects they dealt with were “of so refin’d and spiritual a nature,” that they “must be comprehended by a pure and intellectual view, of which the superior faculties of the soul are alone capable.”¹ Philosophers welcomed this notion, and used it to explain our abstract ideas. But to refute them, Hume declared, one need only remember “that all our ideas are copy’d from our impressions.”² The mathematical idea of a triangle was derived from experience of real triangles, and made abstract in the usual way, which Hume hoped to illustrate by his treatment of space and time. He dismissed also relations of identity and of time and place, because they were comparisons that could be made merely by looking at the two objects. As they involved nothing more than “a mere passive admission of the impressions thro’ the organs of sensation,” the mind had no need to go beyond what was immediately present to the senses.³

      It was different, however, with causation. There something more seemed really to be involved and all our knowledge depended on it. For in making a causal inference, we go beyond direct experience to assert that what was true yesterday and today would also be true tomorrow. All our conclusions about matters of fact beyond observations of what is immediately present depend on assumptions about cause and effect. We assume that because such an object had always been attended with such an effect, other similar objects would be attended with similar effects, and thus regularly go beyond memory and sense: “Should it be said that, from a number of uniform experiments, we infer a connexion between the sensible qualities and the secret powers; this, I must confess, seems the same difficulty couched in different terms. The question still recurs. … Where is the medium, the interposing ideas which join propositions so very wide of each other?”¹ Or, in terms of traditional logic, what is the source of the middle term, the definition that makes our syllogisms about cause and effect possible? Rationalist philosophers explained that the intellect grasped intuitively the relation between cause and effect. Reason, by understanding the nature of the cause, they said, could see the necessity of the effect, or, it could see from the character of the event what had to be the nature of the cause. To make the causal inference, and arrive at knowledge beyond empirical verification, to discover the definitions of things was the acknowledged function of reason.

      Hume was not alone in singling out the ideas of space and time and of causation as peculiarly significant. They had become subjects of general discussion among natural

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