Full Idea
The immediate divisions of that which is are genera, and there will be one science for each genus.
Gist of Idea
The immediate divisions of that which is are genera, each with its science
Source
Aristotle (Metaphysics [c.324 BCE], 1004a04)
Book Reference
Aristotle: 'Metaphysics', ed/tr. Lawson-Tancred,Hugh [Penguin 1998], p.82
A Reaction
This is robustly realist, and right at the heart of Aristotle's philosophy. It explains why essences and forms must be given through genera and differentiae, even though essences are individual. Genera are the only way to identify things.