 There is one important context, however, where Locke does not appear to talk in a conventionalist way about sortal identity, but in a way that seems to be reminiscent of substantial forms. This is when he is discussing the individuation of living things. He understands ordinary bodies as mereological sums of atoms. As such, any change of particle constitutes a new object, for a mereological sum is individuated by its parts and a change of parts means a change of the object constituted by those parts. Treating ordinary, non-living, bodies as complex enduring objects is a matter of convention determined by the concepts we happen to possess. Living things, however, have a deeper principle of unity.