From The Art and Popular Culture Encyclopedia
Self is broadly defined as the essential qualities that make a person distinct from all others. The task in philosophy is defining what these qualities are, and there have been a number of different approaches. The self is the idea of a unified being which is the source of an idiosyncratic consciousness. Moreover, this self is the agent responsible for the thoughts and actions of an individual to which they are ascribed. It is a substance, which therefore endures through time; thus, the thoughts and actions at different moments of time may pertain to the same self (See John Locke's theory of consciousness as the basis of personal identity). As the notion of subject, the "self" has been harshly criticized by Nietzsche at the end of the 19th century, on behalf of what Gilles Deleuze would call a "becoming-other".
Most philosophical definitions of self are expressed in the first person, as with Descartes, Locke, Hume, and William James. A third person definition with which we might all agree does not refer to specific mental qualia but instead strives for objectivity and operationalism.
To another person, the self of one individual is exhibited in the conduct and discourse of that individual. Therefore, the intentions of another individual can only be inferred indirectly from something emanating from that individual.
The particular characteristics of the self determine its identity.