 This basic Aristotelian picture is complicated, if not outright altered, by several very influential fourteenth-century thinkers, including Ockham and Buridan (Normore 2006; Pasnau 2011, 692–702). We will consider Buridan’s theory of identity over time and change (Quaestiones super De generatione et corruptione 1.13; Quaestiones in Physicam 1.10; with English paraphrase in Pluta 2001). According to Buridan, there are three senses of numerical sameness: (1) x is “totally” the same in number as y, (2) x is “partially” the same in number as y, and (3) x is numerically the same as y in a “less proper” way. Something is totally 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 whatsoever persists through mereological change. Something is partially 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. In particular, it is Socrates’ intellective soul which is the principal part and guarantor of persistence through change. Finally, something is less properly the same in number if there is merely a continuous succession of beings that maintain a similar shape, disposition, and form. This less proper mode of numerical sameness allows us to claim that the Nile River here today is numerically the same river as the Nile back in Herodotus’ time. While it is probably not a surprise to learn that rivers are the same in number only in a less proper sense, it is striking that Buridan goes on to assert that plants and non-rational animals too can only be numerically the same in this less proper sense (Quaestiones super De gen. 1.13 (2010, 114–15)). Buridan’s reason for thinking this is that non-rational creatures do not have the sort of soul that can act as a guarantor of less proper identity; rather, non-rational souls are themselves altered when the material parts are altered. Thus, over the span of a normal lifetime, neither the matter nor the form of a horse or an orchid remains numerically the same in anything other than a less proper  sense.[7]