A genus contains its species, and the species implies its genus. Does it mean that the species is a part of the genus and also the genus is a part of the species? But how can they be part of each other without being identical? In the context of kinds, in what sense is ‘part’ applicable?
We argue that for Aristotle, a species and its genus are mutual parts, standing in different parthood relations to each other, viz. the genus is a prior part of its species, while the species is a posterior part of its genus. Furthermore, we show that prior and posterior parthood concerning the genus and its species is grounded on the relations between matter and form in a matter-form compound.