Questioning the obviousness of the indivisibility of the res cogitans, a self-evident truth for Descartes, John Locke took the daring step of proposing that personal identity is grounded in a consciousness of past deeds. Expanding an example first formulated by George Berkeley, Thomas Reid offered the following objection to Locke’s proposal in Essays on the Intellectual Powers of Man, first published in 1785:
“There is another consequence of this doctrine, which follows no less necessarily, though Mr. Locke probably did not see it. It is, that a man may be, and at the same time not be, the person that did a particular action.
Suppose a brave officer to have been flogged when a boy at school, for robbing an orchard, to have taken a standard from the enemy in his first campaign, and to have been made a general in advanced life. Suppose also, which must be admitted to be possible, that when he took the standard, he was conscious of his having been flogged at school, and that when made a general he was conscious of his taking the standard, but had absolutely lost the consciousness of his flogging.
These things being supposed, it follows, from Mr. Locke’s doctrine, that he who was flogged at school is the same person who took the standard, and that he who took the standard is the same person who was made a general. Whence it follows, if there be any truth in logic, that the general is the same person with him who was flogged at school. But the general’s consciousness does not reach so far back as his flogging, therefore, according to Mr. Locke’s doctrine, he is not the person who was flogged. Therefore the general is, and at the same time is not the same person with him who was flogged at school.”
– Thomas Reid, Essays on the Intellectual Powers of Man (Dublin: L. White, 1786), 396-397. https://archive.org/details/
— From the Desk of the Unconscious