Sometimes the attempt was made to reduce the inner to the outer world (Condillac, Mach, Avenarius, materialism); sometimes the outer to the inner world (Descartes, Berkeley, Fichte); sometimes the sphere of the absolute to the others (e.g., by trying to infer causally the essence and existence of something divine in general); […]; sometimes the general differentiation of subject and object to pregivenness of the co- or 'fellow-man', to whom an environmental element—as, for instance, 'this tree' — is supposed to be introjected, followed by subsequent introjection by the observer to himself (Avenarius); sometimes one's own body to a merely associative coordination of the self-perception of the own self and organ sensations with the own body as perceived from outside.
Source: wiktionary