Form
Form refers to order, structure or frameworks that enclose reality into categories and relationships. Where there is no form and no categories, there cannot be words, because words are more or less fixed forms. Where there are no words, there is no communication and hence no growth of knowledge. Therefore, forms are a necessary aspect of reason and knowledge.
Thus Plato argued in defense of his idealism, in which the forms or ideas were fixed "in the heavenly realm" to provide an absolute reference point for reason.
Plato's idealism was moderated somewhat by Aristotle, who agreed that form is a necessary component of reason, but he placed the forms in the natural world, not in the heavens. Nature is a composite (synalon) of form and matter, both of which are complementary aspects of the real world.
With this composite, balanced description of reality as a philosophical foundation, natural science as a rational research program became possible.