The English expression ``self" is a modest one; in its normal use, it is not even quite a word, but something that makes an ordinary object pronoun into a reflexive one: ``her'' into ``herself," ``him" into ``himself" and ``it" into ``itself". The reflexive pronoun is used when the object of an action or attitude is the same as the subject of that action or attitude. If I say Mark Twain shot himself in the foot, I describe Mark Twain not only as the shooter but as the person shot; if I say Mark Twain admired himself, I describe him not only as the admirer but as the admired. In this sense, ``the self" is just the person doing the action or holding the attitude that is somehow in question. ``Self" is also used as a prefix for names of activities and attitudes, identifying the special case where the object is the same as the agent: self-love, self-hatred, self-abuse, self-promotion, self-knowledge.
The phrase ``the self" often means more than this, however. In psychology it is often used for that set of attributes that a person attaches to himself or herself most firmly, the attributes that the person finds it difficult or impossible to imagine himself or herself without. The term ``identity" is also used in this sense. Typically, one's gender is a part of one's self or one's identity; one's profession or nationality may or may not be.
In philosophy, the self is the agent, the knower and the ultimate locus of personal identity. If the thought of future reward or punishments is to encourage or deter me from some course of action, I must be thinking of the person rewarded as me, as myself, as the same person who is now going to endure the hardship of righteousness or pass up the enjoyments of sin in favor of this ultimate reward. But this same self, this same identity, comes up in much more mundane transactions. If I pick up the cake and shove it in this mouth rather than that one, isn't it because I think it will be me, the very same person who picks up the cake, that will have the pleasure of tasting it? This self, the identity of which is at the bottom of every action, and involved in every bit of knowledge, is the self philosophers worry about.
A straightforward view of the self would be that the self is just the person, and that a person is a physical system. This view has been challenged on two fronts. First, the nature of freedom and consciousness has convinced many philosophers that there is a fundamentally non-physical aspect of persons. We shall not dwell on this issue for two reasons. First, the arguments in favor of immateriality of the mind or self do not have as strong a hold on the philosophical community as they once did. While there are many philosophers who think that mental properties cannot be fully reduced to physical or material properties, most such philosophers would allow that these are properties of a physical system, rather than an immaterial self. Second, these issues are considered in articles about the mind-body problem.
The second challenge stems from puzzling aspects of self-knowledge. The knowledge we have of ourselves seems very unlike the knowledge we have of other objects in several ways, and this has led some philosophers to rather startling conclusions about the self. In his Tractatus, Ludwig Wittgenstein tells us that ``I am my world,", and that ```the world is my world'" (Wittgenstein, 1961, 5.63,5.641). This should lead us to the rather surprising conclusion, that I am the world, or that at least Wittgenstein was. He draws at least one conclusion that would follow from this; he says ``...at death the world does not alter, but comes to an end."
The contemporary philosopher Tom Nagel has been led to a possibly less radical but still quite dramatic view. According to Nagel, when he says ``I am Tom Nagel," at least in certain philosophical moods, the ``I" refers to the ``objective self," which is not identical with, but merely contingently related to, the person Tom Nagel. This self could just as well view the world from the perspective of someone else other than him (Nagel, 1983). We need to discuss the puzzling features of self-knowledge that give rise to such views.