Because Plato’s goal is to establish an order by which philosopher rulers can understand not only the immediate world around them but also that unchangeable world of truth which he is held accountable to, he must organize the way we say we know things. Guardians must be philosophers precisely because it is the role of the philosopher to know the highest things, which include both the technic knowledge of the physical world, and the episteme of the unchangeable world. As a means for understanding what the highest things are, he asserts that the ultimate goal of all knowledge is to know the good. This provides him with the framework by which he can assert the forms, those highest known things, as our means for knowing the Good. As a way for us to better understand what he means he presents two ways in which we know things, things of appearance and things that are intelligible. Those two means of knowing can be divided again into two categories. In the things of appearances there are those which we imagine or assert to be something because of its likeness (eikasia), and those of which we believe we see (pistis). The intelligible things can be broken down into those which we think about (dianoia) and those which know (absolutely?) (episteme). The importance of the order is that of immutability. As one progresses from imagination to knowledge what one knows takes on a more permanent form. The goal, it seems, is to remove our tendency to explain things through imagination and sensation (all of which may change) and to instead explain things through ideas and knowledge (which do not change). His Allegory of the cave is a visual example (which this video explains in a fun way) by which we are to understand the disassociation of the intelligible world from the sensible world. Migration to either, he supposes, implies the loss of the ability to immediately understand the other. As soon as we have trained ourselves to know the intelligible things we lose somehow the ability to associate with sensation and visa versa.
Aristotle critiques Plato’s idea that there is a separation between intelligibility and sensation. First, however, he shows how Plato differs from the Pythagoreans — an important point as the Pythagoreans seemed to believe that things themselves were ideas (number), where Plato does not. Aristotle explains that to disassociate the intelligible world from the sensible world is quite certainly maintain that they are independent of each other. We cannot then say that something is a likeness of some form, we can only say that we know this form and we see that thing which it mimics and there is no association between them. Ultimately it seems Aristotle disagrees with Plato because Plato does not admit that there is the efficient cause.
If you would like to explore this topic more thoroughly be sure to take a look at the primary texts. Here are links to the relevant portions of Plato (Republic 502d-521d) and of Aristotle (Metaphysics 987b-988b).