The Platonic doctrine of reality as intelligible form is an expression of the community between thought and being.

Rather than the popular cariacture of stripping the sensible world of all intelligibility and locating it some hypostasised otherworldly realm, Plato expressly presents the forms as the truth, the whatness, the intelligibility, and hence the reality, of the world.

Forms are ideas-in-things in that they are the common content of thought and things, that in things which can also be present in thought.

The single clearest statement in all the Socratic dialogues of why there must be forms occurs in the Parmenides, after an onslaught of arguments seemingly directed against forms, and is placed quite fittingly in the mouth of Parmenides himself:

“if someone will not admit that there are forms of beings--nor mark off some form of each one, he will not have anywhere to turn his thought, as he does not admit a form of each of beings which is always the same, and thus he altogether destroys the power of discourse” Parm.135b5

Without forms, thought and being would be alienated from one another, having no common content. But in that case there would be no thought, since there would be no apprehension of being, and no being, since nothing would be given to thought.

Plato's understanding of the convertibility of being and form can thus be seen as an explication of the Parmenidean dictum, "The same is for thinking and for being."

Is there such a thing as health? Of course there is. Can you see it? Of course not. This does not mean that the forms are occult entities floating "somewhere else" in "another realm". Platonic forms are distinct but not apart from the things themselces.

This imply means that the intelligible identities which are the reality, the whatness, of things are not themselves physical things to be perceived by the senses, but must be grasped by thought, are the natural objects of intellection.