Thus far, I have argued that there is an unconscious of thought at the level of perception and at the level of ideas and desire, as attested to by my readings of Leibniz and Spinoza. But even if one were to allow that there are unconscious perceptions and desires, it might be thought that there is at least one aspect of thought in which consciousness cannot be denied: namely, thinking itself, the very movement of thought, the mental processes by which the mind understands and believes; at least that, one might think, must be a matter of consciousness. But David Hume reformulates the way in which we conceive of the movement of thought: it consists, he tells us, merely in the mind's passage from one idea to another. And it turns out that, in many ways, these passages are neither immediately conscious nor even readily available to conscious reflection: the mind passes from one idea to another on the basis of mental habits that are formed unconsciously, those habituated ideal passages are unconsciously and involuntarily followed, and even direct reflection on them does not clearly reveal the logic of ideal passage or the mechanisms of thinking. There are no beliefs that do not involve these unconscious habits of thought; and Hume argues that nearly every movement of the mind is a matter of belief, rather than knowledge. In short, habits, which are ubiquitous, turn out to be mostly unconscious.

This might come as something of a surprise. After all, what could be better known to the mind than its own operations, its own movements? And yet, Hume teaches us that we hardly understand how the mind works. What we can legitimately do, he suggests, is articulate the movements of thought in terms of what he calls the principles of association: resemblance, contiguity, and causation. The mind seems to pass from one idea to another on the basis of these ideas standing in a relation to one another according to one of these principles: the idea of the Washington monument leads my mind to imagine the obelisks of ancient Egypt; the idea of a street leads my mind to imagine that of the sidewalk; the idea of fire leads my mind to imagine the heat that I believe it must cause.