 Solution

John Locke's primary-secondary quality distinction separates properties such as shape, extension and motion from powers to produce experiences such as colour and taste. On this account a secondary quality is a body's disposition to affect a perceiver through its physical constitution; the disposition should not be confused with the sensation it produces.^[ref]

This distinction can help scientific explanation. A surface's spectral reflectance, the illuminating spectrum, and a visual system's receptor responses jointly affect perceived colour. Altering illumination can change the experience without changing the surface. The lesson is to distinguish an object's physical structure, the surrounding conditions and the observer's response, rather than to assign a colour experience to the object as though no observer or illumination mattered.

A simple absolute contrast between invariant primary properties and observer-dependent secondary properties is less plausible. Special relativity makes measured lengths and durations depend on the inertial frame; appearance of shape also depends on viewing geometry. Dependence on circumstances therefore does not by itself mark a property as unreal. Conversely, a disposition to reflect particular wavelengths or stimulate receptors can be an objective physical relation even though its manifestation requires suitable conditions.

A useful modern version is thus explanatory rather than an unrevisable division of reality. Distinguish independently specified physical variables from response properties defined relative to a system and conditions, then study the mechanism connecting them. The boundary can move with better science. This preserves the insight that experiences require explanation while avoiding the claim that every relational property is merely subjective or that the historical list of primary qualities is final.