Plato's insistence that the eternal immobile model is “the real thing” and the mobile world only an image is to stress the sincerity of his conviction that the intelligible pattern, the unchangeable network of principles, must be the foundation of the physical reality. Only because there is such a fundamentum in re can we have concepts that allow us to understand and explain the world. Without such really existing concepts our thinking would be nothing, it would be a groping for stability in a changing world that could at best provide similarities without any fix point to determine their nature. The concepts that we have are, however, at least for us not separable from the particular applications that we make of them, nor discoverable independently from the experienced world. The application of ‘sameness’ and ‘difference’ in specific ways allows the scientist to work in his field, in the way indicated so often for the musician and writing-instructor. ‘Sameness’ and ‘difference’ are applied in everyday connections all the time. What makes them less stable and precise is the fact that in the realm of becoming the conditions of sameness and difference, first of all, never remain stable (in this respect the same, different in this way) because the objects may constantly change at least in slight ways. Secondly, these conditions can never be fully given (the nuances do not allow us to specify with any precision, for instance, in which way the sound F# is the same as the one I heard a minute ago). The infinity of possible slight variations prevents precision here. Without immutable tools and arid a firm foundation not even relative certainty about the realm of the changeable can be attained.
Frede, Dorothea Aline, "The Philosophical Economy of Plato's Psychology: Common Concepts in the Timaeus" (1990). The Society for Ancient Greek Philosophy Newsletter. 240.