To understand Plato's central philosophy, one must understand the Forms (or "Ideas"). Plato believed that reality is divided into two parts: the ideal and the phenomena. The ideal is the perfect reality of existence. The phenomena is the physical world we experience; it is a flawed echo of the perfect, ideal model that exists outside of space and time.
Plato's theory of ideas is unique in the sense that his theory is the theory of the objectivity of concepts. That the concept, e.g. "man", is not just an idea in the mind, but something that has its own reality, outside and independent of the mind - this is the essence of Plato's philosophy.
So when we ask: "what is a table?", we usually describe only arbitrary attributes, e.g. whether it has 3, 4 or more legs. Nevertheless, we all have an "ideal" idea of a table when we think of a table. This idea is to be understood metaphysically. It exists in the "realm of ideas". Now, it is interesting to note that, according to Plato, we are endowed with fragmentary memories of this world of ideas, since the human soul also comes from the realm of ideas and returns to it after the death of the body.
Thus, a cognition is always an approximation to the eternal, true idea of an object or concept, since the true, the good and the beautiful or justice are also ideas of which we possess a pale glimmer. We can also say that a cognition is thus always a reminder of the metaphysical "ideal" world that we make salient when we deal with a thing. Plato's theory also survives in our winged word of "Platonic love," that is, which takes place without physical exchange and is limited solely to the spiritual-ideal. These ideas have also clearly found their way into the metaphysical theories of Christianity. We speak explicitly of "Neoplatonists" who deal with this synthesis, whose most famous representative is Plotinus.
About the meme there is to say that Aristotle was the disciple of Plato and of course, one cannot see an idea - or can one? Naturally, only if one has not been present in the academy ...
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