The proximity between mathematics and philosophy in Plato is something historically acknowledged and that can be verified from the first contact with his work and with the general lines of his thought. Thus, one can find in some of his main Dialogues, particularly in the Republic, conceptions on the nature of mathematics mainly related to the mathematical methodology. In the Republic Plato approaches critically aspects regarding the method and the epistemological status of the mathematical disciplines in two moments. The first in Book VI, in the famous fragment of the Divided Line (509d - 511e), and the second in Book VII, while describing the program of preparatory studies to dialectics (521c-534e) and, in both cases, considering what Plato says in other fragments, the character of Plato s criticism surprises. In the Line, the disciplines of mathematics are described as a way of knowledge in-between opinion and dialectics, the last being the only one entitled to be considered a legitimate science. In Book VII, in order to show the distinction between the knowledge reached by mathematical disciplines, on one side, and the dialectics, on another, it is stated that despite learning some of the essence, the mathematician is for the dialectical as one who sleeps and dreams is for those who are awake and living reality itself (533b 534e). Therefore, the aim of this work is to investigate why Plato considers the disciplines of mathematics in-between sciences and what notion of knowledge was used as the criteria for that classification.