This work aims to determine what function the Neoplatonic philosopher Proclus (5th century AD) attributes to dialectic in the fifth book of his Commentary on the Parmenides. The first Italian translation of that text is provided, accompanied by an introduction and a commentary. The main characteristics of Proclus' interpretation are discussed, particularly his view of dialectic as a scientific procedure to trace effects to their causes. To do so, dialectic uses the operations of division, definition, demonstration, and analysis. The capstone of these operations is the hypothetical method, a means of establishing what consequences derive from the hypotheses that an object possesses or does not possess a certain property. In Proclus' view, the discovery of this method is Plato's fundamental philosophical achievement, since unlike Aristotle's logic, it has a strong metaphysical significance. This is why it represents the way to attain the cause of everything, i.e. the One.