The philosophy of self defines, among other things, the conditions of identity that make one subject of experience distinct from all others. Contemporary discussions on the nature of the self are not thereby discussions on the nature of personhood, or personal identity. The self is sometimes understood as a unified being essentially connected to consciousness, awareness, and agency (or, at least, with the faculty of rational choice). Various theories on the metaphysical nature of the self have been proposed. Among them, the metaphysical nature of the self has been proposed to be that of an immaterial substance.
Most philosophical definitions of self--per Descartes, Locke, Hume, and William James--are expressed in the first person. A third person definition 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 from something that emanates from that individual. The particular characteristics of the self determine its identity.
In spirituality, and especially nondual, mystical and eastern meditative traditions, the human being is often conceived as being in the illusion of individual existence, and separateness from other aspects of creation. This "sense of doership" or sense of individual existence is that part which believes it is the human being, and believes it must fight for itself in the world, is ultimately unaware and unconscious of its own true nature. The ego is often associated with mind and the sense of time, which compulsively thinks in order to be assured of its future existence, rather than simply knowing its own self and the present.
The spiritual goal of many traditions involves the dissolving of the ego, allowing self-knowledge of one's own true nature to become experienced and enacted in the world. This is variously known as enlightenment, nirvana, presence, and the "here and now".
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For Socrates, the goal of philosophy was to "Know thyself". Lao Tzu, in his Tao Te Ching, says "Knowing others is wisdom. Knowing the self is enlightenment. Mastering others requires force. Mastering the self requires strength."Adi Shankaracharya, in his commentary on Bhagavad Gita says "Self-knowledge alone eradicates misery". "Self-knowledge alone is the means to the highest bliss."."Absolute perfection is the consummation of Self-knowledge."
Aristotle, following Plato, defined the soul as the core essence of a living being, but argued against its having a separate existence. For instance, if a knife had a soul, the act of cutting would be that soul, because 'cutting' is the essence of what it is to be a knife. Unlike Plato and the religious traditions, Aristotle did not consider the soul as some kind of separate, ghostly occupant of the body (just as we cannot separate the activity of cutting from the knife). As the soul, in Aristotle's view, is an activity of the body, it cannot be immortal (when a knife is destroyed, the cutting stops). More precisely, the soul is the "first activity" of a living body. This is a state, or a potential for actual, or 'second', activity. "The axe has an edge for cutting" was, for Aristotle, analogous to "humans have bodies for rational activity," and the potential for rational activity thus constituted the essence of a human soul. Aristotle used his concept of the soul in many of his works; the De Anima (On the Soul) provides a good place to start to gain more understanding of his views.
Aristotle also believed that there were four sections of the soul: the calculative and scientific parts on the rational side used for making decisions, and the desiderative and vegetative parts on the irrational side responsible for identifying our needs.
While he was imprisoned in a castle, Avicenna wrote his famous "Floating Man" thought experiment to demonstrate human self-awareness and the substantiality of the soul. His "Floating Man" thought experiment tells its readers to imagine themselves suspended in the air, isolated from all sensations, which includes no sensory contact with even their own bodies. He argues that, in this scenario, one would still have self-consciousness. He thus concludes that the idea of the self is not logically dependent on any physical thing, and that the soul should not be seen in relative terms, but as a primary given, a substance. This argument was later refined and simplified by René Descartes in epistemic terms when he stated: "I can abstract from the supposition of all external things, but not from the supposition of my own consciousness."
David Hume pointed out that we tend to think that we are the same person we were five years ago. Though we have changed in many respects, the same person appears present as was present then. We might start thinking about which features can be changed without changing the underlying self. Hume, however, denies that there is a distinction between the various features of a person and the mysterious self that supposedly bears those features. When we start introspecting, "we are never intimately conscious of anything but a particular perception; man is a bundle or collection of different perceptions which succeed one another with an inconceivable rapidity and are in perpetual flux and movement".
It is plain, that in the course of our thinking, and in the constant revolution of our ideas, our imagination runs easily from one idea to any other that resembles it, and that this quality alone is to the fancy a sufficient bond and association. It is likewise evident that as the senses, in changing their objects, are necessitated to change them regularly, and take them as they lie contiguous to each other, the imagination must by long custom acquire the same method of thinking, and run along the parts of space and time in conceiving its objects."
On Hume's view, these perceptions do not belong to anything. Rather, Hume compares the soul to a commonwealth, which retains its identity not by virtue of some enduring core substance, but by being composed of many different, related, and yet constantly changing elements. The question of personal identity then becomes a matter of characterizing the loose cohesion of one's personal experience. (Note that in the Appendix to the Treatise, Hume said mysteriously that he was dissatisfied with his account of the self, yet he never returned to the issue.) Hume's position is very similar to Indian Buddhists' conception of the self.
The paradox of the Ship of Theseus can be used as an analogy of the self as a bundle of parts in flux.
Daniel Dennett has a deflationary theory of the "self". Selves are not physically detectable. Instead, they are a kind of convenient fiction, like a center of gravity, which is convenient as a way of solving physics problems, although they need not correspond to anything tangible -- the center of gravity of a hoop is a point in thin air. People constantly tell themselves stories to make sense of their world, and they feature in the stories as a character, and that convenient but fictional character is the self.
The Buddha in particular attacked all attempts to conceive of a fixed self, while stating that holding the view "I have no self" is also mistaken. This is an example of the middle way charted by the Buddha and the Madhyamaka school of Buddhism.