A subject is a unique being that (possibly trivially) exercises agency or participitates in experience, and has relationships with other beings that exist outside itself (called "objects").
In Continental philosophy, 'the subject' is a central term in debates over the nature of the self; it is an 'observer', as distinct from something 'observed'. The nature of the subject is also central in debates over the nature of subjective experience within the Anglo-American tradition of analytical philosophy.
The sharp distinction between subject and object corresponds to the distinction, in the philosophy of René Descartes, between thought and extension. Descartes believed that thought (subjectivity) was the essence of the mind, and that extension (the occupation of space) was the essence of matter.
Subject as a key-term in thinking about human consciousness began its career with the German idealists, in response to David Hume's radical skepticism. The idealists' starting point is Hume's conclusion that there is nothing to the self over and above a big, fleeting bundle of perceptions. The next step was to ask how this undifferentiated bundle comes to be experienced as a unity – as a single subject. Hume had offered the following proposal:
"...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.
Kant, Hegel and their successors sought to flesh out the process by which the subject is constituted out of the flow of sense impressions. Hegel, for example, stated in his Preface to the Phenomenology of Spirit that a subject is constituted by "the process of reflectively mediating itself with itself."
Hegel begins his definition of the subject at a standpoint derived from Aristotelian physics: "the unmoved which is also self-moving" (Preface, para. 22). That is, what is not moved by an outside force, but which propels itself, has a prima facie case for subjectivity. Hegel's next step, however, is to identify this power to move, this unrest that is the subject, as pure negativity.