Plato grammaticus. On the Platonic Concept of epistēmē in the Theaetetus’ Dream-theory”. Theaetetus’ third definition of epistēmē was traditionally labeled as a precursor of the so-called Justified True Belief Knowledge-Model. Nevertheless, Plato’s point in the last section of this dialogue is more elaborated than this classification deems it. I will examine here why and how Plato develops such a strange doctrine as the Dream-theory in order to offer an interpretation of the problem of justification as discussed in the Theaetetus. Thanks to this strategy, I will show that Plato’s Theaetetus aims to prove that the forms are necessary tools not only to justify true beliefs but also, and consequently, to define what knowledge is. The upshots of this analysis with regard to the complex character of the objects of knowledge will lead to a reconsideration of Plato’s theory of knowledge and metaphysics. Additionally, they will result in a better understanding of his contribution on certain epistemological and semantic issues. / Siendo el símil de la línea un mero esbozo, deja muchos temas por esclarecer. El autor intenta echar luces sobre algunos de ellos apoyándose en otros pasajes de la República. Así, propone una correspondencia entre la reflexión sobre la estructura del alma en base al principio de no-contradicción y el segmento de la línea correspondiente a la dianoia, y analiza el curriculum matemático del filósofo para intentar precisar la naturaleza de las imágenes a las que se refiere la eikasia y la naturaleza de los entes matemáticos como objetos de la dianoia.