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Consciousness makes Personal Identity.       §10. But it is farther enquir’d whether it be the same Identical
Substance. This few would think they had reason to doubt of,
if these Perceptions, with their consciousness, always remain’d
present in the Mind, whereby the same thinking thing would be
always consciously present, and, as would be thought, evidently
the same to it self. But that which seems to make the difficulty is
this, that this consciousness, being interrupted always by forget-
fulness, there being no moment of our Lives wherein we have the
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whole train of all our past Actions before our Eyes in one view: But
even the best Memories losing the sight of one part whilst they are
viewing another; and we sometimes, and that the greatest part of
our Lives, not reflecting on our past selves, being intent on our
present Thoughts, and in sound sleep, having no Thoughts at all,
or at least none with that consciousness, which remarks our waking
Thoughts. I say, in all these cases, our consciousness being inter-
rupted, and we losing the sight of our past selves, doubts are raised
whether we are the same thinking thing; i.e. the same substance
or no. Which however reasonable, or unreasonable, concerns not
personal Identity at all. The Question being what makes the same
Person, and not whether it be the same Identical Substance, which
always thinks in the same Person, which in this case matters not at
all. Different Substances, by the same consciousness (where they do
partake in it) being united into one Person; as well as different
Bodies, by the same Life are united into one Animal, whose
Identity is preserved, in that change of Substances, by the unity of
one continued Life. For it being the same consciousness that makes
a Man be himself to himself, personal Identity depends on that only,
whether it be annexed only to one individual Substance, or can be
continued in a succession of several Substances. For as far as any
intelligent Being can repeat the Idea of any past Action with the
same consciousness it had of it at first, and with the same conscious-
ness it has of any present Action; so far it is the same personal self.
For it is by the consciousness it has of its present Thoughts and
Actions, that it is self to it self now, and so will be the same self as far
as the same consciousness can extend to Actions past or to come;
and would be by distance of Time, or change of Substance, no more
two Persons than a Man be two Men, by wearing other Cloaths to
Day than he did Yesterday, with a long or short sleep between: The
same consciousness uniting those distant Actions into the same
Person, whatever Substances contributed to their Production.
Locke Hum II, 27, §10, pp. 335-336