Since the ancient Greek philosophers (esp. Plato and Aristotle), the question what a universal is, whether it exists ontologically or only epistemologically,
and what its relation is to particulars, has occupied the mind of philosophers. Below, I summarize the main theories.
In Plato, the universal as an abstract entity has its own independent spiritual being. The ideas are the actual being and constitute the reason for
being and knowledge for the material single things. For Plato true knowledge is only possible of something immutable. The things perceptible by