"...And as I observed that this truth 'I think, therefore I am' (Cogito ergo sum) was so certain and of such evidence ...I concluded that I might, without scruple, accept it as the first principle of the Philosophy I was in search."
"...In the statement 'I think, therefore I am' ... I see very clearly that to think it is necessary to be, I concluded that I might take, as a general rule, the principle, that all the things which we very clearly and distinctly conceive are true..."
While reading Descartes, Locke began to relish the great ideas of philosophy and the scientific method. On one occasion, while in a meeting with friends, the question of the "limits of human understanding" arose. He spent almost twenty years of his life on the subject until the publication of An Essay Concerning Human Understanding, a great chapter in the History of Philosophy.
John Locke's chapter XXVII "On Identity and Diversity" in An Essay Concerning Human Understanding (1689) has been said to be one of the first modern conceptualizations of consciousness as the repeated self-identification of oneself, through which moral responsibility could be attributed to the subject—and therefore punishment and guiltiness justified, as critics such as Nietzsche would point out, affirming "...the psychology of conscience is not 'the voice of God in man'; it is the instinct of cruelty ... expressed, for the first time, as one of the oldest and most indispensable elements in the foundation of culture." John Locke does not use the terms self-awareness or self-consciousness though.
According to Locke, personal identity (the self) "depends on consciousness, not on substance" nor on the soul. We are the same person to the extent that we are conscious of our past and future thoughts and actions in the same way as we are conscious of our present thoughts and actions. If consciousness is this "thought" which doubles all thoughts, then personal identity is only founded on the repeated act of consciousness: "This may show us wherein personal identity consists: not in the identity of substance, but ... in the identity of consciousness." For example, one may claim to be a reincarnation of Plato, therefore having the same soul. However, one would be the same person as Plato only if one had the same consciousness of Plato's thoughts and actions that he himself did. Therefore, self-identity is not based on the soul. One soul may have various personalities.
Self-identity is not founded either on the body or the substance, argues Locke, as the substance may change while the person remains the same: "animal identity is preserved in identity of life, and not of substance", as the body of the animal grows and changes during its life. Take for example a prince's soul which enters the body of a cobbler: to all exterior eyes, the cobbler would remain a cobbler. But to the prince himself, the cobbler would be himself, as he would be conscious of the prince's thoughts and acts, and not of the cobbler's life. A prince's consciousness in a cobbler body: thus the cobbler is, in fact, a prince. But this interesting border-case leads to this problematic thought that since personal identity is based on consciousness, and that only oneself can be aware of his consciousness, exterior human judges may never know if they really are judging—and punishing—the same person, or simply the same body. In other words, Locke argues that you may be judged only for the acts of your body, as this is what is apparent to all but God; however, you are in truth only responsible for the acts for which you are conscious. This forms the basis of the insanity defense: one can't be held accountable for acts in which one was unconsciously irrational, mentally ill—and therefore leads to interesting philosophical questions:
[...] personal identity consists [not