What is most real, universals or particulars? Plato and Aristotle gave diametrically opposed answers to this basic ontological question and proceeded to build their entire philosophies on the epistemological foundation of their individual answers. As a result, the question has been debated for centuries, some siding with Plato, others with Aristotle, and others still attempting to syncretize the ideas of each. For Plato, universal Forms or ideas are most real. They are imperceptible to the senses, but all that the senses perceive are, according to Plato, a mere shadow of the Forms. Plato sees the universal Forms as ontologically prior to the individual particulars perceived by the senses. His pupil Aristotle, on the other hand, not only affirms that individual particulars are ontologically prior to universals, but that without the individual particulars, there would be no universals. For Aristotle, individual particulars are most real.
In the Categories, Aristotle asserts that substances are, “in the truest and primary and most definite sense of the word” (2a11-13) individual particulars, such as this computer or that telephone (2a13). He asserts that only individual particulars are “neither predicable of a subject nor present in a subject” (2a12-13). Aristotle explains what being “predicable of” or “present in” a subject means in chapter two of the Categories. Being predicable of a subject or not, is simply a distinction between abstract universals and concrete particulars. The particular man, Codell Carter, is not predicable of any subject, but the universal “man” is predicable of Codell Carter. My particular copy of A First Course in Logic is not predicable of any subject, but the universal term “book” is predicable of my particular copy of A First Course in Logic. In other words, universal terms are predicable of subjects, while particular terms are not. As for being or not being in a subject, this refers to the possibility of independent existence. Aristotle describes a subject as that which is “incapable of existence apart from the said subject” (1a22-23). Thus, that which is not in a subject is the subject itself. Since the “Arctic Silver” color of my BMW, for example, cannot exist separately from my BMW, said color is in a subject. But the BMW itself is not in any subject, since the BMW is the subject (3a10-16). Thus, Aristotle distinguishes between attributes (which are present in a subject) and entities (which are not present in a subject) (Hsieh).