If formal unity were the only unity belonging to the nature, there would be no basis for asserting that humanity is just one species rather than many.

Fonseca is convinced of the reality of haecceities because he accepts the view that common natures have in themselves a certain sort of unity prior to instantiation. Fonseca proposes that, in order to have real existence, something needs to be added to the nature – a haecceity (Fonseca, In Met. V, c. 28, q. 3, sect. 4 [Fonseca (1599), vol. 2, col. 966A]). Again, Fonseca’s discussion is informed by the criticisms of Cajetan, though in this case the position that Cajetan adopts on the question of