As adults we have little difficulty thinking of ourselves as mental beings extended in time. Even though our conscious thoughts and experiences are constantly changing, we think of ourselves as the same self throughout these variations in mental content. Indeed, it is so natural for adults to think this way that it was not until the 18th century-at least in Western thought-that the issue of how we come to acquire such a concept of an identical but constantly changing self was first recognized as a problem that required an explanation. Philosophical discussion of this issue was initiated when John Locke (1694/1975) proposed a notion of personal identity and selfhood based on consciousness:

For since consciousness always accompanies thinking, and ‘tis that, that 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 ‘tis by the same self with this present one that now reflects on it, that that Action was done. (p. 335)

According to this view, we are the same self insofar as we can consciously accept as our own not only those mental and physical acts that we perform now but also those acts done in the past, that we can recollect as our own. This Lockean view of self based on consciousness was an advance beyond Descartes’s (1641/1984) notion that the self was an immaterial soul or mind, whose identity was guaranteed not by being conscious of itself but by being a substance distinct from the body.