From The Art and Popular Culture Encyclopedia
A theory of substantial forms asserts that there are things called forms (or ideas), and that they are what organize matter and make it intelligible. Substantial forms are the source of order, unity, and identity of objects.
The idea of substantial forms dominates ancient Greek philosophy and medieval philosophy, but has fallen out of favour in modern philosophy. The idea of substantial forms has been abandoned for a mechanical, or “bottom-up” theory of organization.