 Within this material domain, a material substratum underlies all change, and real change typically involves the gain or loss of some form. This can be made to look straightforward enough when examples are carefully chosen. If a tree’s limbs are bent under the weight of an early fall snowstorm, then the tree undergoes alteration (accidental change) in virtue of taking on a new shape (a new accidental form), while the material substance (the tree itself) endures. If the storm in fact kills the tree, then the substance is corrupted (a substantial change), in virtue of losing its substantial form, which is the internal principle that gives the substance its nature. In cases like this, where the substance is destroyed, Aquinas advances a metaphysical claim that is quite surprising: that all that endures through the change is the most basic material substratum, prime matter, which lacks all form. (Aquinas’s early On the Principles of Nature summarizes the basic story. For general discussions of Aquinas’s metaphysics of material bodies see Wippel 2000 chs. 7–9, Brower 2014. On causation at the level of creatures see Frost 2022.)