If we compare together the various passages in which Aristotle cites and applies the Ten Categories (not merely in the treatise before us, but also in the Metaphysica, Physica, and elsewhere), we shall see that he cannot keep them apart steadily and constantly; that the same predicate is referred to one head in one place, and to another head in another: what is here spoken of as belonging to _Actio_ or _Passio_, will be treated in another place as an instance of _Quale_ or _Ad Aliquid_; even the derivative noun [Greek: e(/xis] (_habitus_) does not belong to the Category [Greek: e)/chein] (_Habere_), but sometimes to _Quale_, sometimes to _Ad Aliquid_.[104] This is inevitable; for the predicates thus differently referred have really several different aspects, and may be classified in one way or another, according as you take them in this or that aspect. Moreover, this same difficulty of finding impassable lines of demarcation would still be felt, even if the Categories, instead of the full list of Ten, were reduced to the smaller list of the four principal Categories--Substance, Quantity, Quality, and Relation; a reduction which has been recommended by commentators on Aristotle as well as by acute logicians of modern times. Even these four cannot be kept clearly apart: the predicates which declare Quantity or Quality must at the same time declare or imply Relation; while the predicates which declare Relation must also imply the _fundamentum_ either of Quantity or of Quality.[105] [Footnote 104: Aristot. Categor. p. 6, b. 2; p. 8, b. 27.]