Most medieval philosophers paid lip service to the maxim that the whole is ontologically posterior to its parts (cf. Boethius, De div. 879C). Although most, then, duly, restricted this claim to cover only some parts - the essential or principal parts. Very few medieval philosophers flirted with forms of mereological essentialism.
But the occasional philosopher did seem to endorse the thesis that a whole depends upon each one of its parts. Abelard, for example, insisted that every integral whole is composed of a unique set of integral parts (Henry 1991:92-151). This house must be composed out of these nails, these boards, and this cement. Given that each whole is composed of a unique set of parts, if any part is removed, that whole is destroyed. Another whole similar to the original might exist after the mereological change takes place, but strictly speaking the two wholes would not be identical. Some later medieval nominalists, such as Ockham and Buridan, also argued that even minute mereological changes bring about the destruction of most integral wholes, including nonrational animals (Normore 2006:751-753). According to these nominalists there are three senses of numerical sameness: there is a proper sense, a less proper sense, and an improper sense. Something is properly the same in number if all its parts remain the same and it neither acquires nor loses any parts. In this strictest of senses, no corruptible thing persists through a change of a material part. Something is less properly the same in number if its ‘‘most principal part’’ remains numerically the same. This is the sense that allows us to claim that Socrates is numerically the same man now as that man ten years ago, since Socrates’ intellective soul persists through the change (Buridan, In Metaphys. VII, q. 12, 48va). Finally, something is improperly the same in number if there is a continuous succession of beings that maintain a similar shape, disposition, and form. This improper mode of numerical sameness allows us to claim that the Nile River here today is numerically the same river as the Nile back in Caesar’s time. The nominalists claimed that plants and animals can only be numerically the same in the third, improper sense, for these creatures do not have the sort of soul that can act as a guarantor of less proper identity.
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