According to Plato, what we see, hear, taste, feel, or smell is subject to constant change and
therefore expressed in mere opinion, while true knowledge is of what is stable and unchanging. For
example, we can see water freezing, knowing it's still being water, an individual changing a lot over
the years, still being the same person, and so on. To have true knowledge is to have an infallible
knowledge of the real, and the real can be grasped only in a clear, universal definition, not be opinion.
We have to ask about the "essence" of things.
In The Republic,Plato sets forth the simile of the line by which he divides all knowledge into the realm
of opinion and the realm of true knowledge. While opinion relates to particulars (for example, an
individual horse), knowledge relates to universals (the essence of horses, the "horseness", that is
applicable in all cases, the norm of the particular horses). For Plato, opinions can be shaken by
criticism or by conflicting evidence, while true knowledge cannot. In the Republic, he seeks to
illustrate his meaning by distinguishing four grades of cognition, each with its own class of objects.
The lowest grade is that of mere guesswork (eikasia), which has as its objects the images of dreams
or the reflections in water. A higher state of cognition is that of belief pistis), where one has learned to
distinguish physical things from their mere shadows. Here a person has a conviction about the
experience of the world as known through the senses. It is only when we move higher, to
understanding (dianoia), that we have knowledge— when we move, so to speak, from a particular
horse to the essence, "horseness," that which makes all horses alike as horses but different from
human beings and other animals. There is, however, one more step needed to ascend to the
supreme first principle (noesis). Each step in the ascent to knowledge moves to a higher level of
abstraction, farther and farther from the particular and more and more toward the universal: from the
shadow of a horse to a specific horse to horseness to the basic and fundamental principles
characteristic of all biological life.