Plato thought the table in your kitchen is only a shadow
For Plato, the chair you sit on and the horse in the field are less real than their Forms: perfect, changeless essences that exist beyond space and time. Every physical object only imitates its Form. Oddly, Plato's own dialogues sometimes attack the theory as vigorously as they defend it.
The puzzle Plato inherited was about change. The pre-Socratic thinkers noticed that appearances shift and asked what a changing thing really is underneath. Their answer was substance, whatever persists beneath the change. Thales, the earliest of them on record, said everything is ultimately water. Pythagoras went a step further towards Plato, holding that the essence of things is abstract rather than physical, but for him those essences were specifically numbers.
Plato cast the net much wider. Every kind or quality, from justice and equality to beds and horses, has its own Form. A Form answers the question of what something essentially is. There are countless different tables, but each counts as a table because it shares in the single Form of tableness, a kind of perfect blueprint. Objects of the senses are fleeting copies, forever in flux, while Forms are unqualified perfection. That makes the theory both realist, since certain ideas really exist, and idealist, since reality is ultimately made of ideas.
The vocabulary is visual. Plato's words for Forms include idea, from a root meaning to see, and eidos, a visible shape, and he used the two interchangeably. Highest of all stands the Form of the Good. Perhaps his most original conclusion concerns where Forms are: nowhere and never. An ideal isosceles triangle does not change with time or place, so Forms lie outside space and outside time altogether, lacking location, as a geometric point does.
Plato presents all this only through characters, mainly Socrates. Some passages claim Forms are the only things that can be truly known rather than merely believed; others question whether Forms exist at all. Philosophers still argue over the theory, but it remains a classic answer to the problem of universals, the question of what different things share when we call them by the same name.
Source: Theory of forms
