We often meet a large number of objects which we can call as “particulars” for our convenience. For example, cows, dogs, tables etc are so many particulars. Now each class of particulars have some “common qualities”. While some of these qualities can be called as essential, the others can be called as accidental.
Let’s take an example to understand this. If we take cow as an example, while the color of the particular (cow) may be white or black, the universal “cowness” of all the cows belonging to the class cow is something that is an essential. The color, or for that matter the size (small or large cow), may be accidental, but this “cowness” will always be an essential quality.
No animal which doesn’t have the universal essential quality of “cowness” can be a cow. Hence, this cowness becomes/is said to be universal and cows comprising the class cow are said to be particulars.
Plato calls such Universals as Ideas and maintains that they have an independent objective existence. This doctrine of considering the universals to have an independent existence is called Realism. These independent ideas can be copied by/in a perceptible but no perceptible can be considered an original idea. This is analogous to Pythagorean geometry. Example : Consider the concept of equality. In sensory experience, no two objects can be exactly equal, but the idea of equality, nonetheless, has its own worth.
Plato’s view of Realism is opposed by Aristotle. According to Aristotle, universals are not the things that exist independently, but in individual things! (Meaning Aristotle considers that it wasn’t the universals having an independent existence rather it was the particulars that had an independent existence, and that universals existed in the particulars.) He uses example to explain this. He says that : “Beauty (universal) doesn’t exist. It is the beautiful things(particulars) that exist, viz beautiful flowers, birds, butterflies etc.” In other words, according to Aristotle, a universal must be instantiated
Conceptualism is the doctrine that universals are constructed by the human mind after the observation of particular instances. This is believed by Socrates, which basically means that ideas/universals are mental constructs and concepts. According to conceptualism, universals have their locus in the mind. This view is rejected by Plato as it leads to subjectivity of universals. Not only this, but even Aristotle rejects it on the ground that ideas/universals are not merely mental constructs or concepts, but they also have their existence, albeit not independent of particulars.
How Plato reconciles Heraclitus and Paramenides :
Both of them have made sharp distinctions between the worlds of Being and Becoming. According to them, reality is to be known through reason and not through senses. Heraclitus propounded that becoming or transience is the only reality of the world. Everything in the world is in a constant state of flux. Eternity is a mere illusion. He gave the analogy that we cannot bath in the same river twice.
On the other hand, Paramenides propounded that being or eternity is the only reality and that change is a mere illusion.
Plato reconciles these two theories by accepting two realities or worlds, known as Transcendental Dualism. According to Plato, there are two kinds of worlds. One is the sensory world of our existence, which is empirical. This world is in constant state of flux and continuously changing. On the other hand, there exists a TRANSCENDENTAL world of IDEAS, of which the SOUL is also a part, where eternity is the ONLY reality. Thus, both being and becoming are parts of human understanding.