indifferentia

Indifferentia is a term from Aristotelian and medieval logic referring to the distinguishing attribute that defines a species within a genus. In traditional syllogistic, a term’s definition is often expressed as genus plus indifferentia (also called differentia or differentiating feature). The indifferentia serves to specify what makes the species different from other species within the same genus.

The function of indifferentia is to carve out the essence of a species. It should be essential and non-accidental, capturing a core characteristic that all members of the species share but that sets the species apart from other species of the genus. For example, in the classical definition “man is a rational animal,” animal is the genus and rational is the indifferentia, defining the species man within the genus animal.

Historically, indifferentia plays a central role in the Porphyrian tree and the five predicables—genus, species, difference (indifferentia), property, and accident. The indifferentia is distinguished from accidental properties in that it is part of what makes a term essentially what it is, rather than a contingent attribute that may or may not obtain without changing the term’s fundamental identity.

In modern logic and philosophy, the concept survives as the idea of an essential defining attribute used to distinguish a term within a broader class. While the technical vocabulary has evolved, the underlying notion of genus plus distinguishing attribute remains foundational to how definitions were historically constructed in classical syllogistic. See also Porphyry, Aristotelian logic, syllogism, and the predicables.