According to Derrida, the most fundamental notions of Western thought--that is, the notions of metaphysics, the branch of philosophy that tries to contemplate the deepest ground, the first causes of existence--are based on illusions. The illusion that dominates Western thought, and that therefore is deconstruction's target, is the ideal of presence. All Western philosophy, according to Derrida, strives to reach a fundamental level where truth and meaning are fully present. All philosophical attempts at definition, at indicating the determining grounds for something, the principle on which something is based--all these attempts imply the ideal of presence. They all imply that, if one could only go (back) deep or far enough, one could clearly determine the essence, the 'pure meaning of something. This ideal of metaphysical essences expressed in perfect, pure definitions is an illusion, an impossible dream, according to Derrida.
However, at the same time, it is an impossible dream from which we cannot free ourselves, without which our language would not be able to function, argues Derrida. Seen by themselves, words seem nothing more than a series of marks or sounds, 'without life,' one could say; a word seems to require something that accompanies it, that is 'present' to it and, as such, gives meaning to that word. Derrida says that we necessarily regard a word as a supplement for something else, as referring to something-to a thing in the world, or a thought in my head. Without that connection, a word would appear to be dead, meaningless.
By presupposing that a word functions as a supplement to something, as referring to something outside itself, a gap opens up between language and what it seeks to express in the world (for example, an object, or a thought in my head). If that gap is to be bridged, if language is to express the world, a clear and unequivocal connection between language and world is required, that infuses words with accurate meaning, and thereby, their capacity to describe the world. However, such an accurate reflection of language and world requires a shared metaphysical 'origin', a system of essences, of transcendental signifieds that underlies both the being of the world and the possibility for its accurate expression in language.