Constancies

Constancies refer to the quality or state of remaining the same under varying conditions. The term is used across disciplines to describe properties or parameters that do not change—or are treated as not changing—within a given theoretical framework. The plural form highlights the existence of several distinct kinds of unchanging properties studied in different fields.

In perception, constancies are perceptual processes that preserve stable interpretations of the environment despite changing sensory input. Examples include color constancy (colors perceived as the same under different illumination), brightness or lightness constancy (objects seen as having constant brightness despite shadows or highlights), and size, shape, or orientation constancy (recognizing an object as the same size, shape, or orientation as viewing conditions vary).

In science, constancies appear as physical constants and mathematical invariants. Physical constants, such as the speed of light or Planck’s constant, are assumed to be fixed across space and time within physical theories. In mathematics and physics, invariants refer to quantities that remain unchanged under specified transformations, providing a foundation for laws and symmetries.

Philosophically, constancy raises questions about persistence and identity: what it means for an object, a person, or a property to remain the same through change, and how continuity of identity is defined or demonstrated.

In statistics and research methodology, constancy can describe assumptions of stationarity or parameter stability—conditions under which models remain valid because underlying processes do not systematically vary over time or across conditions.

See also constancy, invariance, stationarity, physical constant.