III.D9: Aversion Defined

Aversio est tristitia concomitante idea alicujus rei quæ per accidens causa est tristitiæ. De his vide scholium propositionis 15 hujus.

Aversion is a sadness accompanied by the idea of some thing which is a sadness by accidental cause. On these see IIIP15S. 

Consistent with the discussion on tendency, Spinoza is arguing another behavior is rooted in the primitives - this time sadness. For centuries, people have debated the idea of boys loving, and girls hating spiders and snakes. Spinoza is arguing that this "accidental" perception is rooted in an unwillingness to acknowledge the cultivation of the emotions driving, in this case, aversion.