the limits of its kind or species_, are what we call qualities. They diversify the substance accidentally in its perfection, in its concrete mode of existing and behaving: by their appearance and disappearance they do not change the _essential perfection_ of the substance (46), they do not effect a substantial change; but they change its intermediate, accidental perfection; and this qualitative change is technically known and described as _alteration_(324) (11). Hence we find _Quality_ described by St. Thomas as the sort of accident which modifies or disposes the substance in itself: “_accidens modificativum sen dispositivum substantiaein seipsa_,” and by Albertus Magnus somewhat more explicitly as “the sort of accident which completes and perfects substance in its existence and activity: _accidens complens ac perficiens substantiam tarn in existendo quam in operando_”.(325) This notion will be conveyed with sufficient clearness if we describe _Quality_ as _that absolute accident which determines a substance after the manner of an accidental _“differentia,”_ affecting the essential perfection of the substance in regard to its existence or to its activity_. Hence (1) the Pure Actuality of the Infinitely Perfect Being cannot admit qualities, inasmuch as quality implies only a relative and limited perfection; (2) the qualities of a corporeal substance are grounded in the _formative_ principle which gives that substance its specific nature and is the principle of its tendency and development towards its final perfection, whereas its _quantity_ is grounded in its determinable or _material_ principle; (3) the _essential_ differentiating principles of substances—being known to us not intuitively, but only abstractively and discursively, _i.e._ by inference from the behaviour of these substances, from the effects of their activities—are often designated not by what constitutes them intrinsically, but by the _accidental perfections_ or _qualities_ which are our only key to a knowledge of them. For instance, we differentiate the nature of man from that of the brute beast by describing the former as _rational_: a term which really designates not the essence or nature itself, but one of its fundamental qualities, _viz._ the faculty of reason.