Impressions and ideas

David Hume (1711–1776) opens A Treatise of Human Nature (1739) and the later Enquiry Concerning Human Understanding (1748) with a bold ambition: to be the Newton of the mind. Just as Newton found a few simple laws governing the physical world, Hume wants a science of human nature — a small set of principles governing how the mind actually works, built by observation rather than speculation. Everything in his epistemology grows from the very first move he makes.

That first move is a division. Every perception of the mind — every mental content whatsoever — falls into one of two classes, differing only in their force and vivacity (their liveliness):

• Impressions: our original, forceful experiences — sensations, passions, emotions as first felt. The heat of a fire, the taste of a lemon, a stab of anger.
• Ideas: the faint copies of impressions that we use in thinking, remembering, and reasoning. Recalling the fire later, or reading the word anger.

The difference is not what they are about but how vivid they are. An idea is a kind of dimmed echo of an impression.

Hume now refines the point with a distinction between simple and complex perceptions. Simple perceptions admit no separation into parts (a particular shade of red); complex ones are built out of simples (an apple = that red + that roundness + that taste). His central law — the Copy Principle — says: every simple idea is a copy of a corresponding simple impression, resembling it exactly and derived from it.

Now, the golden mountain. Complex ideas can be genuinely novel recombinations — the mind is free to rearrange its materials, joining gold and mountain into something it never perceived. But it cannot create a simple element from nothing. Break the golden mountain into its simples — the colour gold, the shape of a mountain — and every piece traces back to some impression you really had. Imagination shuffles the deck; it cannot print new cards.

Here is why the Copy Principle matters so much: it becomes Hume’s test of meaning, his microscope. Whenever a philosophical term looks suspicious — substance, self, necessary connection — Hume poses one devastating question: “From what impression is this idea derived?” Trace it to an impression and it is legitimate. Find no impression behind it and the supposed idea is empty — a word with no genuine content. Hume will aim this weapon at the grandest claims of the rationalists.

Hume is scrupulous enough to flag one apparent counterexample: the missing shade of blue. Suppose someone has seen every shade of blue but one, arranged from lightest to darkest with a gap where the missing shade belongs. Shown the series, could they form the idea of that unseen shade? Hume thinks they probably could — the mind fills the gap. This is a simple idea with no prior impression, a direct violation of his own law. Remarkably, he admits it, calls it “so particular and singular” that it is not worth altering the principle for, and moves on.

One last piece of the machinery. If ideas are the faint copies, how do they get organised into the flowing trains of thought we actually experience? Hume answers that ideas are linked by three natural relations — gentle forces of association that lead the mind from one idea to the next:

• Resemblance — a portrait leads the mind to its subject.
• Contiguity in time or place — thinking of one room prompts the next.
• Cause and effect — a wound brings to mind the pain.

These are, he says, “a kind of attraction” in the mental world — his deliberate echo of Newtonian gravity. The third relation, cause and effect, will turn out to be the most important idea in all of Hume’s philosophy.