 In addition to unities of nature (substances), Descartes recognizes “unities of composition”, for instance, the flesh and bones of the same animal-machine (CSM 2.285, 157; AT 7.423, 222). Other examples are a mouse, a human body and a human being. Such things are ens per se: although they have parts that can exist outside them, the parts are then regarded as incomplete. So the mind/body unity is robust enough to tag a mind separated from its body as incomplete (To Regius, December 1641; CSM 3.200; AT 3.461). Brown and Normore (2019: 52–60) contend that things with this status have a nature of the type Descartes calls “true and immutable” (hereafter “TIN”). Anything with a TIN has ground in reality indicated by its being the subject of demonstrable truths; corporeal nature, geometrical entities and God are mentioned in Meditation 5 (CSM 2.44–5; AT 7.63–5) and it is said to be true of Cartesian animals (Conversation with Burman, 16 April 1648; CSM 3.143–4; AT 5.160). The bi-conditional test for things with a TIN is having a property that arises from the conjunction of all its ingredients (CSM 2.84; AT 7.118; Brown &amp; Normore 2019: 55–60). That is, the composite possesses some property that does not already belong to one of its parts, as in the case of Pegasus. For instance, a triangle inscribed in a square has a property the triangle does not have on its own, namely, having an area half that of the square that superscribes it (CSM 2.83–4; AT 7.117–18). Descartes maintains that the union of mind and body has a TIN which would, if known, account for a human being’s having appetites, emotions, and sensations of colors, etc. (CSM 1.208–9; AT 8.23).