Substance and personal identity

Locke’s empiricism has a built-in stress test. If every idea comes from experience, what about the idea of substance — the thing that has qualities but is not itself any of them? We never sense a bare “thing-in-itself”; we only ever encounter the qualities. So where does the idea of an underlying stuff come from?

Substance as substratum. Locke’s answer is candid. Finding a bunch of simple ideas that constantly go together — the particular colour, weight, malleability, and shininess we call gold — we can’t imagine how they’d subsist on their own, so we suppose some substratum underlying them, in which they inhere and from which they result. We accustom ourselves to assume a support (Essay II.xxiii.1–2).

But — and this is the striking admission — we have no clear idea of this substratum at all. Pressed to say what the support is, we can only say it is “something we know not what” (Essay II.xxiii.2). Our idea of substance-in-general is purely negative and relative: a something we posit merely as the support of qualities, of whose positive nature we are wholly ignorant.

This connects to Locke’s honest map of the limits of knowledge. Of any substance we grasp only its nominal essence — the observed cluster of qualities (the abstract idea by which we sort and name it: what makes something count as “gold”). We do not grasp its real essence — the inner, corpuscular constitution from which those qualities actually flow. Because that hidden constitution is closed to us, most of our knowledge of the world is not certain and demonstrative but merely probable (Essay III.vi; IV).

Now Locke’s most influential and radical move — personal identity (added in the 1694 second edition, Essay II.xxvii). He first insists we ask the identity question relative to a kind. The identity of a man (a living human organism) is one thing — sameness of continued life, like a plant or animal. But the identity of a person is a different question entirely, and answering it requires knowing what “person” means.

A person, Locke says, is “a thinking intelligent being, that has reason and reflection, and can consider itself as itself, the same thinking thing, in different times and places” (Essay II.xxvii.9). The key is self-consciousness: a person is a being aware of its own thinking, that can regard itself as the same self stretched across time. This definition, not biology, sets the identity condition.

The consciousness criterion. So what makes a person at time 2 the same person as one at time 1? Not sameness of soul-substance, and not sameness of body or organism — but continuity of consciousness. Personal identity “extends itself beyond present existence to what is past, only by consciousness.” As far as your consciousness can be extended backward to any past action or thought — as far as you can, in effect, remember it as your own — so far reaches your identity as a self (Essay II.xxvii.9–10).

Locke leans on further cases to force the point. The sleeping and the drunk man: if a man does something while blackout-drunk and has genuinely no consciousness of it, then on Locke’s strict criterion the sober man is — in the eyes of the theory — not the same person who acted (though human law rightly punishes him, since courts can’t see into consciousness). And a soul reincarnated into a new body with no memory of a past life would not make you the same person as whoever had that soul before. Same substance, different person.

Why does Locke build identity this way? Because “person” is, he says, a forensic term — a concept belonging to law, morality, and accountability. It “appropriates” actions to a self and grounds responsibility, praise, and punishment. It is just that you answer for a deed only if it is yours — connected to you by consciousness. Divine justice, Locke notes, will hold each to account for exactly what his consciousness owns, and no more. Identity is tied to what you can rightly be held answerable for (Essay II.xxvii.18, 26).