A quality (from Lat. qualitas) is an attribute or a property. Attributes are ascribable, by a subject, whereas properties are possessible. Some philosophers assert that a quality cannot be defined. In contemporary philosophy, the idea of qualities and especially how to distinguish certain kinds of qualities from one another remains controversial.

Background

Aristotle presented his idea of qualities in his Categories. According to him, qualities may be attributed to things and persons or be possessed by them. There are four Aristotelian qualities: habits and dispositions, natural capabilities and incapabilities, affective qualities and affections, and shape.

Locke presented a distinction between primary and secondary qualities in his An Essay Concerning Human Understanding. For Locke, a quality is an idea of a sensation or a perception. Locke further asserts that qualities can be divided in two kinds: primary and secondary qualities. Primary qualities are intrinsic to an object—a thing or a person—whereas secondary qualities are dependent on the interpretation of the subjective mode and the context of appearance.For example, a shadow is a secondary quality. It requires a certain lighting to be applied to an object. For another example, consider the mass of an object. Mass is intrinsic to an object since it relies solely on the amount and type of atoms contained, thus making it a primary quality. Weight is a secondary quality since, as a measurement of gravitational force on said collection of atoms, it varies depending on the distance to, and mass of the Earth (or other body) as described by Newton's law of universal gravitation.

Specific qualities related to philosophy include qualia and quality of life.