 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 the unity of the common nature seems closer to the broadly Thomist and Scotist tradition than that proposed by Fonseca. In other words, Fonseca adopts an account of the common nature that seems quite distinct from the closely related views of Aquinas and Scotus. To this extent, Fonseca’s account represents an acceptance of haecceities in the context of a theory of universals that is not really Scotist. According to Cajetan, common natures in themselves have “formal” unity: indivisibility into further more specific kinds, compatible with division into numerically many instances or subjective parts (Cajetan [CBE], nn. 134–155 [nn. 55–62]; see Fonseca, In Met. V, c. 28, q. 3, sect. 4 [Fonseca (1599), vol. 2, col. 967B, D-E]). Fonseca agrees that the nature must have this formal unity – a unity that he identifies as Scotus’s less-than-numerical unity (Fonseca, In Met. V, c. 28, q. 3, sect. 4 [Fonseca (1599), vol. 2, col. 968A]). But, in addition to this unity, he holds that the nature has a proper unity of its own that cannot be communicated to its instantiations. This unity is numerical, and belongs to the nature absolutely (unqualifiedly) (Fonseca, In Met. V, c. 28, q. 3, sect. 4 [Fonseca (1599), vol. 2, col. 968A]). The reason is that, if a nature lacked such additional unity, then it would not be possible to speak of (say) humanity as just one species. After all, the formal unity of the nature is compatible with division into numerically many instances; some other unity must be the basis for the assertion that humanity is not many natures or species (Fonseca, In Met. V, c. 28, q. 3, sect. 1 [Fonseca (1599), vol. 2, col. 959F]). The picture is admittedly not entirely clear, but Fonseca’s distinction of instantiable from non-instantiable unity is presumably supposed to explain how there can be some unity proper to common natures; this unity is distinguished from instantiable unity, and it is the existence of haecceities that explains how the instantiable unity is in fact instantiated.