Plato's theory of forms can be summed up as follows:
1) All things are physical manifestations of forms
2) Knowledge is only ever knowledge of the forms; beliefs are of the material world
3) The highest form is that of the good
The good has two roles:
- epistemological; it allows us to know of relativism (e.g. justice, cleanliness, etc.) and therefore of all things (e.g. one knows what a table is by what a good table is)
- ontological; nothing can exist without the good, without some form from which is is manifest. The good also allows relative properties to exist
Plato distinguishes knowledge (forms) from beliefs (material world):
1) Knowledge is of what is
2) Belief is of what is and what is not
3) The world consists of what is and what is not
The world consists of objects of belief
4) Forms are things which are
Forms are objects of knowledge
(The Republic, 476d-480a)
This represents a distinction similar to Descartes', in that it asserts that knowledge is only of those things about which we are absolutely certain, and those truths aren't of objects in the material world.
Point three only works if the conclusions are accepted; 'what is not' must be held to be an inaccurate perceptual relation to a form, whilst 'what is' must be an accurate perceptual relation to a form. 'What is not' could equally be thought of as an inaccurate perceptual relation to an object held in the mind.
This problem forces us to ask what sense of "to be" Plato is referring to. We have three senses of "to be" in Western philosophy and in English:
existential - what can exist in some way / form
veridical - what is true
predicative - what is a certain way / what has a certain property
Plato cannot be talking of the predicative sense of being since a sensible object can then neither be nor not be, e.g. an object can be neither big nor not big
Nor can he be talking of the veridical sense of being, since truth is binary, and Plato wants the disctinction to allow for relativism of the kind made possible by The Good. In this case, either object A is a good table or it is not a good table; so therefore all tables that exist are good tables, whilst bad tables must be imaginary or illusory.
Plato must be referring to the existential sense of being. But how can one make sense of degrees of existence, neither existing nor not existing, but existing in some way? Perhaps it is not that forms necessarily exist, but that if anything exists, forms must exist, i.e. forms are a necessary condition of existence, and define the various ways in which sensible objects can exist. So we can have knowledge of that which is necessary for the existence of that which we can perceive, and these objects of knowledge neither are nor are not coming in or going out of existence. In that sense, forms can also be immune from change, as Plato suggests, since they have no spatial or temporal properties.