The second category, that of property [187] or form, comprises all those essential attributes, by means of which a definite character is impressed on matter otherwise indeterminate. [188] If the definite character be one which belongs to a group or class, it is called a common quality—κοινῶς ποιόν·—or, if it be something peculiar and distinctive, it is called a distinctive quality—ἰδίως ποιόν. [189] Properties therefore combined with matter constitute the special materials out of which individual things are made; [190] and quality in this combination (ποιόν), corresponds, as Trendelenburg has well shown, [191] with the form (εἶδος) of Aristotle. [192] It may, in fact, like that, be described as the active and efficient part of a thing. [193] Aristotle’s form, however, expresses only the non-material side of a thing, whereas quality is regarded by the Stoics as something material—in fact, as an air-current. [194] Hence the mode in which a quality is conceived to reside in matter is that of an intermingling of elements. [195] The same theory of intermingling applies of course to the union of several properties in one and the same matter, [196] and likewise to the combination of several attributes to produce a single conception of quality. [197] In all cases the relation is supposed to be materialistic, and is explained by the doctrine of the mutual interpenetration of material things. [198] This explanation, indeed, could not apply to every kind of attributes. Unable to dispense entirely with things not material, [199] the Stoics were obliged to admit the existence of attributes belonging to immaterial things, these attributes being, of course, themselves not material. [200] What idea they formed to themselves of these incorporeal attributes, when reality was considered to belong only to things corporeal, it is, of course, impossible for us to say. [201] [(δ) The categories of variety and variety of relation.]