The notion of metaphor undergoes a series of reconsiderations within the aesthetic works of Gaston Bachelard. In this study we will consider only a very small part of his broad production on rêverie (1938–1948). The aim of this contribution is to show how the function of the metaphor can’t in any way be limited to a rhetorical expedient. In fact, it represents a very powerful manifestation of the imaginative faculty. As proof of this thesis, we will consider Bachelard’s reflections on the active function of the metaphor, on the reversibility of the metaphor and on the relationship between metaphor and metamorphosis.