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Theory-ladenness can affect the concepts used to describe an observation, the interpretation of an instrument and even which features attract attention. A detector's pulse height is not already a particle's energy: the inference depends on measurement calibration, response assumptions and background subtraction. This defeats a particularly strong empiricism according to which every scientific claim rests on a completely theory-free observational foundation.

It does not follow that observation can never constrain theory. The crucial distinction is between relying on some background account and presupposing the truth of the particular target under test. One can calibrate a detector with independently characterized sources and compare measurements from detectors with different systematic errors. Agreement across these routes is more compelling than repeated use of one disputed reduction procedure. Scientific replication and tests with deliberately varied apparatus can expose shared assumptions rather than merely repeat them.

An empiricist can therefore adopt a fallible, public evidential practice. The observable phenomenon need not be linguistically uninterpreted to have an evidential role. Reports can be translated sufficiently for disagreement, and raw records can be retained so that rival analyses can be compared. Where all purported evidence is defined through the target theory itself, the circularity is a real problem; the reply must supply independent checks, not simply assert objectivity.

Constructive empiricism offers one further distinction: accepting a theory for its empirical adequacy does not require believing its full story about every unobservable entity. Its proponent can use a theoretical account to organize instruments without identifying use with unconditional belief.^[ref] Theory-ladenness therefore undermines an infallible foundation, while leaving room for constrained, corrigible empiricism.

The distinction supplies a coherent strategy for an empiricist realism, but does not by itself justify that realism. It separates an object's relatively stable physical constitution from its effects on perceivers. The strongest defensible version concerns the explanatory roles of properties, rather than a division between wholly real properties and wholly unreal sensations.

For John Locke, primary qualities include extension, shape, solidity, number and motion; secondary qualities are powers of bodies to produce experiences such as colours and tastes through their constitution. The experienced colour need not resemble the physical cause of that experience. Importantly, a power belonging to a body is not thereby imaginary: the primary and secondary qualities distinction need not deny that an unobserved object has dispositions to affect perceivers.^[ref]

This helps empiricism accommodate perceptual variation without abandoning an external world. A surface can produce different colour experiences under different illumination while keeping approximately the same spectral reflectance. A warm and a cool hand can give different sensations in the same water; this need not mean that the water has two incompatible temperatures. The observer's initial state belongs in the explanation. Controlled changes in illumination, viewing conditions and bodily state allow repeatable predictions about experience. An empiricist can therefore treat physical objects as explanatory sources of regularities in experience, rather than demand that every appearance be copied into the object.

However, this explanatory success does not establish that ideas of primary qualities resemble mind-independent properties. We cannot step outside every perceptual and instrumental relation to compare an experience directly with its alleged original. Moreover, the motion and arrangement of unobserved constituents are themselves theoretical commitments. If empiricism permits only what is immediately given in sensation, the distinction introduces precisely the inference beyond sensation that requires defence. If empiricism permits fallible explanatory inference, scientific realism becomes possible, but the justification comes partly from that additional account of inference.

George Berkeley's challenge is that perceived extension and shape vary with viewpoint and perceiver just as colour does, and that extension cannot be experienced wholly apart from sensible qualities.^[ref] This undermines the simple test “variable appearance means merely secondary”. It does not prove that stable physical properties are absent. An obliquely viewed circular disc can produce an elliptical image; geometrical optics explains the relation between the disc and that image. Variation can be evidence for a systematic relation, rather than evidence against an external cause.

Modern physics also discourages treating Locke's list as an unrevisable inventory. Special relativity makes measured length depend on the relation between an object and an inertial frame; motion was relational even in Principle of Galilean relativity. Neither fact makes the measurements arbitrary. Conversely, spectral reflectance is an objective, measurable property even though perceived colour depends on illumination and the perceiver. Objectivity, relationality, perceptual variability and explanatory fundamentality are distinct issues.

The plausible distinction is consequently between an experience and the physical properties and relations that explain it, with the explanation open to correction. An empiricist realism can use this distinction to explain why appearances change while a world persists. It cannot obtain a proof of the external world, or a privileged catalogue of intrinsic properties, simply by naming some qualities primary. The gain is an empirically disciplined model of perception; its limit is that the realist interpretation still needs an argument.