Hegel, and later Sartre (but from a very different perspective), emphasized the importance of The Other, in the sense of the definition of the Self.

In short, by defining the other, one demarcates the boundary of the self and defines ones identity. Self-identity necessitates the other, in its self-conception and interdependence of the latter's existence. To be reductionist, what does it even mean to be oneself if there is no other?