 The best answer that twelfth-century thinkers come up with is that a principal part of x is that which, when removed, compromises the form of x (Pseudo-Joscelin De generibus §§ 23–6 (2014, 130–31); Arlig 2013, 106–10). Many later medieval philosophers followed suit. At least in the case of substances, the substantial form could do the metaphysical work of guaranteeing persistence through time and change (Pasnau 2011, 689–92). For example, we have already seen that Burley draws a distinction between the whole secundum formam and the material whole (section 3.1). The whole considered formally persists so long as the form persists. The whole considered materially is the only whole compromised by changes in material parts (De toto et parte, 301). Most commonsense objects are identified with a whole considered with respect to form, and so there would be no reason to think that these things are substantially compromised by material changes. (The fate of artifacts, would be different, as their forms are merely accidental forms. A mereological change in the case of an artifact would seem to entail that the initial artifact is replaced by another one. But later medieval philosophers do not seem to be all that troubled by these implications. Whether theologians could stomach all the consequences that this view of artifacts entails (for example, for whether something remains ritually pure after it has been chipped) may be another matter. But this is an area whether further research is needed.) 