In philosophy, a subject is a being which has subjective experiences, subjective consciousness or a relationship with another entity (or "object"). A subject is an observer and an object is a thing observed. This concept is especially important in Continental philosophy, where 'the Subject' is a central term in debates over human autonomy and the nature of the self.
The sharp distinction between subject and object corresponds to the distinction, in the philosophy of Rene Descartes, between thought and extension. Descartes believed that thought (subjectivity) was of the essence of the mind, and that extention (the occupation of space) was of the essence of matter.
In the modern continental tradition, which may plausibly be said to date from Descartes, debates over the nature of the Subject play a role comparable to debates over personhood within the distinct Anglo-American tradition of analytical philosophy.
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, pgph. 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. Subjective self-motion, for Hegel, comes not from any pure or simple kernel of authentic individuality, but rather, it is
The Hegelian subject's modus operandi is therefore cutting, splitting and introducing distinctions by injecting negation into the flow of sense-perceptions. Subjectivity is thus a kind of structural effect - what happens when Nature is diffused, refracted around a field of negativity and the "unity of the subject" for Hegel, is in fact a second-order effect, a "negation of negation". The subject experiences itself as a unity only by purposively negating the very diversity it itself had produced. The Hegelian subject may therefore be characterized either as "self-restoring sameness" or else as "reflection in otherness within itself" (ibid.)
Jacques Lacan, inspired by Heidegger and Saussure, built on Freud's psychoanalytic model of the subject, in which the "split subject" is constituted by a double bind: alienated from jouissance when he or she leaves the Real, enters into the Imaginary (during the mirror stage), and separates from the Other when he or she comes into the realm of language and difference in the Symbolic or the Name of the Father.
Thinkers such as Althusser, Foucault or Bourdieu theorize the subject as a social construction. According to Althusser, the "subject" is an ideological construction (more exactly, constructed by the "Ideological State Apparatuses").
It is constituted through the process of interpellation; according to Foucault, it is the "effect" of power and "disciplines" (See Discipline and Punish: construction of the subject as student, soldier, "criminal", etc.).