The ego is an ‘imaginary function' formed primarily through the subject's relationship to their own body. The subject, on the other hand, is constituted in the symbolic order and is determined by language. There is always a disjunction, according to Lacan, between the subject of enunciation and the subject of the utter-ance; in other words, the subject who speaks and the subject who is spoken.
Lacan argued that the ‘I' in speech does not refer to anything stable in language at all. The ‘I' can be occupied by a number of different phenomena: the subject, the ego or the unconscious. For example, in what Lacan called ‘empty speech', the ‘I' would correspond to the ego; in ‘full speech' it corres-ponds to the subject; while at other times it corresponds to neither subject nor ego. This is what Lacan means when he says I is an other, that is to say, ‘I' is not ‘me'; these two terms do not refer to the same entity; the subject is not the same as the individual person – it is de-centred in relation to the individual. In short, Lacan de-essentializes the ‘I' and prioritizes the symbolic, the signifier, over the subject. It is the structure of language that speaks the subject and not the other way around. Lacan summarizes this in his famous statement, the subject is that which is represented by one signifier to another. The seminar on The Purloined Letter is nothing less than an exposition of this, whereby the subject is caught up in the chain of signification and it is the signifier that marks the subject, that defines the subject's position within the symbolic order.