 In keeping with the tradition, Twardowski calls the matter of an object the sum of its material parts and the form of an object the sum of its formal parts. A special kind of primary formal parts are (relations of) properties (Eigenschaften): these are relations between an object as a whole and one of its material parts, consisting in the whole's having the part at issue (1894, §10, 56). Since Twardowski accepts that among the parts of an object there are the relations in which that object is in, Twardowski's mereology is, strictly speaking, an atomless mereology: there are no simples (1894, §12, 74). However, we can speak of atoms if we restrict ourselves to material parts only (for instance, in the case of the number one, we can say it is a simple object only if we consider the (proper) material parts it has, namely zero).