Key Arguments

• Locke defines 'person' explicitly as 'a thinking intelligent being, that has reason and reflection, and can consider itself as itself, the same thinking thing, in different times and places.'
• He argues that this self-consideration is achieved 'only by that consciousness which is inseparable from thinking, and, as it seems to me, essential to it: it being impossible for any one to perceive without perceiving that he does perceive.'
• He notes that whenever we 'see, hear, smell, taste, feel, meditate, or will anything, we know that we do so', and 'by this every one is to himself that which he calls self', regardless of whether 'the same self be continued in the same or divers substances.'
• From these premises he concludes: 'For, since consciousness always accompanies thinking, and it is that which makes every one to be what he calls self, and thereby distinguishes himself from all other thinking things, in this alone consists personal identity, I.e. the sameness of a rational being.'
• He then states his famous criterion: 'as far as this consciousness can be extended backwards to any past action or thought, so far reaches the identity of that person; it is the same self now it was then.'

Source Quotes

Personal identity. This being premised, to find wherein personal identity consists, we must consider what person stands for; — which, I think, 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; which it does only by that consciousness which is inseparable from thinking, and, as it seems to me, essential to it: it being impossible for any one to perceive without perceiving that he does perceive. When we see, hear, smell, taste, feel, meditate, or will anything, we know that we do so.

When we see, hear, smell, taste, feel, meditate, or will anything, we know that we do so. Thus it is always as to our present sensations and perceptions: and by this every one is to himself that which he calls self:— it not being considered, in this case, whether the same self be continued in the same or divers substances. For, since consciousness always accompanies thinking, and it is that which makes every one to be what he calls self, and thereby distinguishes himself from all other thinking things, in this alone consists personal identity, I.e. the sameness of a rational being: and as far as this consciousness can be extended backwards to any past action or thought, so far reaches the identity of that person; it is the same self now it was then; and it is by the same self with this present one that now reflects on it, that that action was done.

Thus it is always as to our present sensations and perceptions: and by this every one is to himself that which he calls self:— it not being considered, in this case, whether the same self be continued in the same or divers substances. For, since consciousness always accompanies thinking, and it is that which makes every one to be what he calls self, and thereby distinguishes himself from all other thinking things, in this alone consists personal identity, I.e. the sameness of a rational being: and as far as this consciousness can be extended backwards to any past action or thought, so far reaches the identity of that person; it is the same self now it was then; and it is by the same self with this present one that now reflects on it, that that action was done. 10.

Key Concepts

• we must consider what person stands for; — which, I think, 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;
• which it does only by that consciousness which is inseparable from thinking, and, as it seems to me, essential to it: it being impossible for any one to perceive without perceiving that he does perceive.
• and by this every one is to himself that which he calls self:— it not being considered, in this case, whether the same self be continued in the same or divers substances.
• For, since consciousness always accompanies thinking, and it is that which makes every one to be what he calls self, and thereby distinguishes himself from all other thinking things, in this alone consists personal identity, I.e. the sameness of a rational being:
• and as far as this consciousness can be extended backwards to any past action or thought, so far reaches the identity of that person; it is the same self now it was then;

Context

Book II, chapter XXVII, section 9, where Locke introduces his influential definition of 'person' and argues that the identity of a person over time is constituted by the continuity of consciousness, not by sameness of substance.