It is now standard to interpret Aquinas as recognizing two main types of material objects: substances and artifacts, where substances are those material objects that result from some particular substantial form inhering in prime matter, and artifacts are those material objects that result from some particular accidental form inhering in one or more material substances. There are two problems with this standard interpretation. First, there are passages in which Aquinas states that accidental forms should be understood not as inhering in (...) substances from the outside, but as entering into their composition so as to be included among their metaphysical parts. Second, there are passages in which Aquinas states that it is impossible for any accidental form to be shared by two or more substances. This article considers what implications these two observations might have for how we understand the metaphysical structure of artifacts in Aquinas’s ontology. (shrink)