The Background of David Hume

David Hume was a Scottish philosopher who, after suffering a breakdown in his late teens and early twenties during a period of intense private study, recovered and as a precocious youth wrote A Treatise of Human Nature, published in 1739–40. It would fall “dead born from the press.” The writer would then re-establish himself as a famous historian and rewrite his Treatise in a much shorter but also less radical form.

The Philosophy

In the earlier work the emphasis is much more on ideas. These are copies of the world that become linked to each other through similarity or being circumstantially related in space and time. Such ideas then form strong interactions with each other. In consequence the pattern of these ideas, as they are triggered, creates a burst of consciousness within the mind. We believe a match will light because the idea of flame has fused strongly with the idea of striking. This general phenomenon Hume called habit or custom, and its implication is that we not only learn from the past but are actively shaped by it in the present.

Once an impression triggers this chain of ideas we experience meaning as a process of signification, as one idea after another comes to mind in a brief instant. Repetition of this experience makes the ideas fuse more strongly and we start to construct actual laws of cause and effect. We associate heat with a flame through their constant conjunction and our consciousness flows imperceptibly between those two ideas. The place of this activity is the Imagination and as Hume stated in An Enquiry Concerning Human Understanding, its manner is a kind of magical power in the soul.

The ideas themselves appear to trigger other ideas as though they themselves were combusting or undergoing a chemical change. For Hume it is the passions, roused by this vivid train of ideas, that can then cause action through the will, whilst reason, normally a primary faculty for philosophers, plays only an assisting role.

David Hume the Visionary

There is also an interesting connection to modern reinforcement psychology that Hume illuminates, when he speaks of ideas gaining “force and vivacity” through repetition, although he leaves the notion of degrees of vivacity somewhat unresolved. In short he is unspecific whether this term names a felt intensity or something more objectively causal in directing the mind and it is a question later commentators have never fully settled.

Nevertheless, Hume claims that one idea leads to another attendant idea with increasing force through repetition. I have wondered what shape this reinforcement takes. If a stronger connection causes the mind to linger longer on an idea, and lingering longer in turn strengthens the connection further, the rate of reinforcement becomes proportional to the connection already present. For any keen mathematician, this is, formally, the condition for exponential growth. Read this way, Hume’s account contains the seed of a self-reinforcing, almost combustive process of habit-formation, in which early repetitions compound quickly upon each other.

But such growth cannot continue unchecked, and Hume’s own picture implicitly requires it not to. A fully formed habit settles into steady expectation rather than escalating indefinitely. This vigorous reinforcement early on, tapering as the habit approaches some natural ceiling fits both Hume’s descriptions of settled belief and, remarkably, the trajectory found in modern computational models of habit formation, where habit strength has been shown to move through an early phase of rapid, near-exponential reinforcement before transitioning into a different, saturating regime as the habit becomes entrenched. Hume had no such experimental apparatus available to him and never ventured a mathematical account of “vivacity’s” growth. But the qualitative shape he traced, with accelerating reinforcement giving way to settled habit, anticipates, in outline, a pattern that twentieth and twenty-first century habit-science would later have the tools to formalise.