 The resulting picture seems to postulate, as the constituents of substances, three types of basic entities: prime matter, substantial forms, and accidental forms. How exactly to understand each of these has been subject to a great deal of discussion over the centuries. With respect to accidental forms, one of the chief puzzles is to define those cases in which we have a genuine change of form. Aquinas clearly thinks, for instance, that colors and other proper sensibles are accidental forms. But does he think, as the above example of the tree implied, that shapes too are accidental forms? It is not entirely clear, and depends on difficult questions about how he understands Aristotle’s theory of the  categories  (Wippel 2000 ch. 7; Pasnau 2011 ch. 12). With respect to substantial form there is an analogous problem arising from unclarity over what exactly the substances are (Pasnau 2002 ch. 3; Rota 2004; Marmodoro and Page 2016). He clearly takes all living things to be substances, although there are puzzles here about how to define the beginning and end of life (Van Dyke 2012; Amerini 2009 [2013]). When it comes to nonliving things the situation is still less clear. He clearly thinks that artifacts like a house are not substances. Water is a substance, but it is not clear whether a puddle of water is a single substance or a collection of substances. Aquinas does not discuss these issues in the sort of detail that later medieval authors would, and as a result there is little scholarly consensus about his views. But one important guide to his thinking about substances is that they have unity in a very strong sense: “whatever is one in substance is one absolutely (simpliciter)” (ST 1a2ae 17.4c). Of course, no creature has the sort of unity that God has; God’s perfect simplicity precludes even the composition of essence and existence, which is found in all created substances, even in immaterial angels (Wippel 2000 ch. 5). Material substances, being composites of form and matter, have less unity than the angels. Even so, Aquinas thinks there can be complex material substances, unified by their substantial form. How does a substantial form unify? An important part of his answer is that a substantial form actualizes not only the whole substance, but each part: “a form of the whole that does not give existence to the individual parts of the body, … such as the form of a house, is an accidental form” (ST 1a 76.8c).