In the paper, the Plethon’s criticism over the exclusion of the case of perceptible-and-perceptor relations from the principle of necessary compatibility of correlatable under the category of relation by Aristotle is considered. Plethon proceeds from Plato’s assumption on that the Universe is a unique created perpetual living being, a certain hierarchically arranged whole of parts (common and particular). This constitutes a basis for Plethon to state that the principle of coexistence of correlatable, as different from Aristotle, has no exclusions according to neither being by actuality, nor being by potentiality and acts like the basic definition of the whole.
     

Plethon, Plato, Aristotle, category of relation, unity of being, sensory perception, perceptible