According to Husserl, a distinction has to be made between generalizing and formalizing abstraction.² By means of generalizing abstraction one arrives at notions such as "red," "color," "sensuous quality," or "triangle," "planimetric figure," "spatial form," etc. Each of these notions refers to what Husserl calls a "material region"—that is to say, a region circumscribed and defined by a certain qualitative content of whatever pertains to it. If, therefore, an object, a fact, a phenomenon, can be considered as an instance of one of these notions, it is because of its material and qualitative nature. Each of the notions in question expresses an invariant with regard to variations and varieties. Starting from a certain red, for example, one can vary, if only in imagination, its brightness; one can even vary its shade or hue while taking care that all the resulting variations remain red.