Quality / Quantity — Two of the fundamental categories with which philosophy describes being. In Aristotle they appear among the ten categories set out in the treatise Categories: quality (from the Greek ποιόν, poión, “of what kind”) designates the way something is — white, hot, virtuous, just — whereas quantity (ποσόν, posón, “how much”) designates how much something is, covering number (discrete quantity) and continuous magnitude such as extension and duration. For Aristotle it is proper to quality to admit of degrees of more and less and to ground relations of likeness, while quantity grounds relations of equality. In Modernity the distinction shifts inside quality itself: Galileo, in The Assayer (Il Saggiatore), and Descartes reduce physical reality to what is measurable, separating primary qualities — extension, figure, number, motion, taken to be objective and inherent in the body — from secondary qualities — color, sound, taste, smell — regarded as subjective effects produced in the observer. Locke fixes this terminology in his Essay Concerning Human Understanding (1689), making it a pillar of classical physics and of the modern debate on perception. The opposition returns transformed in Hegel, for whom the gradual accumulation of quantitative changes culminates in a leap that alters the quality of a thing — the so-called conversion of quantity into quality, tied to his notion of measure (Maß).
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