Although we are capable of separating and combining our simple ideas as we please, there is, nevertheless, a regular order to our thoughts. If ideas occurred to us completely randomly, so that all our thoughts were “loose and unconnected”, we wouldn’t be able to think coherently (T 1.1.4.1/10). This suggests that
There is a secret tie or union among particular ideas, which causes the mind to conjoin them more frequently, and makes the one, upon its appearance, introduce the other. (Abstract 35)
Hume explains this “tie or union” in terms of the mind’s natural ability to associate certain ideas. Association is not “an inseparable connexion”, but rather “a gentle force, which commonly prevails”, by means of which one idea naturally introduces another (T 1.1.4.1/10).
In the first Enquiry, Hume says that even though it is obvious to everyone that our ideas are connected in this way, he is the first philosopher who has “attempted to enumerate or class all the principles of association” (EHU 3.2/24). He regards his use of these “universal principles” as so distinctive that he advertises them as his most original contribution—one that entitles him call himself an “inventor” (Abstract 35).
Hume identifies three principles of association: resemblance, contiguity in time and place, and causation. When someone shows you a picture of your best friend, you naturally think of her because the picture resembles her. When you’re reminded of something that happened in the 1960s—miniskirts, for example—you may think of the Vietnam War, because they are temporally contiguous. Thinking of Sausalito may lead you to think of the Golden Gate Bridge, which may lead you to think of San Francisco, since they are spatially contiguous. Causality works both from cause to effect and effect to cause: meeting someone’s father may make you think of his son; encountering the son may lead you to thoughts of his father.
Of the three associative principles, causation is the strongest, and the only one that takes us “beyond our senses” (T 1.3.2.3/74). It establishes links between our present and past experiences and our expectations about the future, so that “all reasonings concerning matters of fact seem to be founded on the relation of Cause and Effect” (EHU 4.1.4/26). Taking aspirin in the past has relieved my headaches, so I expect that the aspirin I just took will soon relieve my present headache. Hume also makes clear that causation is the least understood of the associative principles, but he tells us, “we shall have occasion afterwards to examine it to the bottom” (T 1.1.4.2/11).
Like gravitational attraction, the associative principles are original, and so can’t be explained further. Although the associative principles’ “effects are everywhere conspicuous” their causes “are mostly unknown, and must be resolv’d into original qualities of human nature, which I pretend not to explain”. Accordingly, we should curb any “intemperate desire” to account further for them, for doing so would take us illegitimately beyond the bounds of experience (T 1.1.4.6/12–13).
Hume doesn’t try to explain why we associate ideas as we do. He is interested only in establishing that, as a matter of fact, we do associate ideas in these ways. Given that his claim that the associative principles explain the important operations of the mind is an empirical one, he must admit, as he does in the first Enquiry, that he cannot prove conclusively that his list of associative principles is complete. Perhaps he has overlooked some additional principle. We are free to examine our own thoughts to determine whether resemblance, contiguity, and causation successfully explain them. The more instances the associative principles explain, the more assurance we have that Hume has identified the basic principles by which our minds work.
In the Abstract, Hume concludes that it should be “easy to conceive of what vast consequences these principles must be in the science of human nature”. Since they “are the only ties of our thoughts, they are really to us the cement of the universe, and all the operations of the mind must, in great measure, depend on them” (Abstract 35). Just what these “vast consequences” are will become clear when we examine Hume’s revolutionary accounts of our causal inferences and moral judgments.