 The task, then, is to determine what kind of parts form and matter are, and especially what kind of parts substantial form and matter are. Aquinas routinely distinguishes between the “quantitative” parts of a thing and the form and the matter, which he calls “parts of the essence” (Summa Theologiae I, q. 8, a. 2, ad 3; I, q. 76, art. 8; III, q. 90, a. 2). However, he does not provide a clear answer to the question whether the part of an essence is a type of integral part, or whether it is a distinct kind of part. Other philosophers present a less ambiguous line: only parts that make up some quantity can be integral parts (see, e.g., Lambert of Auxerre Logica 126; Peter of Spain Tractatus V.14 (2014, 212–4); Albert of Saxony Sophismata 45). Accordingly, many medieval thinkers will propose that in addition to potential, subjective, and integral parts, there is a fourth kind of part, which I will call a substantial  part.[4]