Out of the immense bibliography on Pedro Calderón de la Barca's La vida es sueño has emerged a relatively small number of studies that have analyzed the influence of Plato's philosophy in this play. However, little attention has been given specifically to the image of the cave that Calderón and Plato both use in their works. The purpose of this study is to show that Calderón does not simply transplant his grotto onto the Spanish stage, but rather inverts its imagery to renovate the myth of the cave according to the aesthetics and philosophy in vogue during the seventeenth century in Spain. The playwright does this in La vida es sueño by turning the cave into the locus of desengaño and enlightenment wherein he clearly sees the artificiality of the world, which then leads him to liken it to a stage. In La hija del aire, Semíramis comes to this realization as well after experiencing similar injustices. However, unlike the Polish prince, she uses this knowledge in sinister ways to take over Babylon. By comparing the caves found in La vida es sueño and La hija del aire with the backstage dressing room in El gran teatro del mundo, we find that these grottoes and the vestuario function similarly, in that they first trap their inhabitants in its "cradle," which keeps them from "acting" in the light of the world-stage, and, later, unlike in Plato's allegory, serve as a space of knowledge in the form of a "sepulcher" that reveals to them the theatricality of the world. Therefore, the juxtaposition of these two comedias reveals that it is not knowledge alone that saves the individual, but rather the way s/he "acts" upon acquiring it.