Barthes, "What is Criticism?" in Critical Essays (1963): "The object of criticism is not 'the world' but a discourse, the discourse of someone else: criticism is discourse upon a discourse; it is a second language, or a metalanguage (as the logicians would say), which operates on a first language (or language object)" (258).
Quoted by Stephen Heath form La théorie (interview) in VH 101, 2, Summer 1970, p. 9, in Vertige du déplacement, lecture de Barthes, Paris, Fayard, 1974, p. 79 (translated by Robert Young):
"Theory, precisely if one conceives of it as a permanent autocritique, ceaselessly dissolves the signified which is always ready to reify behind science."