The human is an animal that refers to itself as an "I". According to Descartes, the subject is an axiom, and everything else follows from this primordial certainty. This is a dualism: to postulate an I as separate from the natural world. Prodi rejects this dualism. The challenge of Prodi is to find a naturalistically way to explain how human subjectivity can emerge from the world of things; that is, from biosemiotic complementarity to the I. For Prodi, following Vygotsky's hypothesis, the "I" qua self-conscious psychological entity, is inseparable from the pronoun "I", i.e. the discursive capacity to refer to oneself. Human consciousness is therefore the capacity to pay attention to oneself by means of language.