In my thesis I first discussed the problem of the subject which arises in language and the contingency of identity as the consequence of a split subject. What interested me the most were the structuralist and psychoanalyst aspects of the split subject. The structuralist aspect allowed me to think about the subject as a place in a structure, which keeps constantly reinstalling itself and by doing that, disables the possibility of constistuting the bedrock for a coherent identity. This structure is the structure of language where the subjectivity of the one who talks is observed in temporary and random points which are a direct consequence of a speech act. The failure of identity is shown in the unconscious as well and here one cannot simply overlook the psychoanalytic point of view. When entering the symbolic and the order of language, the subject arises, but so does the lack which disables the success of any kind of promised identification. The subject is an "I" only for someone else, for an other, ever since entering the symbolic and precisely because of this entry, and is as such not capable of coherent identification. The findings of the first part of the thesis were then applied to the phenomenon of heteronomy in the work of the Portuguese poet Fernando Pessoa. As a modernist poet, he used the establishment of heteronyms and proved himself to be innovative in the poetic expression of the split subject. The realization of the incapacity of identity was poetised through the other or at least, in the name of the other. Each of the heteronomies writes their poetry as an "I," with a wholesome worldview expressed in it, and by that achieves the autonomy of its own "I" and its authorship. Pessoa uses the concept of heteronomy as a way of writing and by that expresses the understanding of identity as the highest level of differentiation.