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Hardness is no more automatically a primary quality than redness. The comparison turns on whether one means an experienced sensation, a disposition to affect a perceiver, or a physically specified property of a material. Keeping those three levels distinct supports a useful distinction without treating either tactile or visual experience as a direct copy of an object's constitution.

John Locke's list of primary qualities includes extension, figure, number, motion and solidity. A secondary quality is instead a body's power, grounded in its constitution, to produce a sensory experience. The colour experience does not literally resemble the shape or motion of the particles producing it. Crucially, Lockean solidity is not hardness. In the former account, both water and a diamond exclude another body from the space they occupy; they differ in how readily their parts move aside. In book II, chapter IV, section 4, John Locke explicitly treats ordinary judgments of hard and soft as relative to the human body's response. Thus his own distinction does not warrant simply assigning everything felt by touch to the primary class.^[ref]

A tactile example makes the distinction intelligible. The same surface may resist a fingertip yet yield to a harder indenter. Its measured hardness also depends on the load, test geometry and material state. Its resistance is nevertheless reproducible under specified conditions. Similarly, perceived color depends on the illuminating optical spectrum, the surface's reflectance and the perceiver's color vision. A change of illumination can change an appearance without changing the surface. In neither case does dependence on specified conditions make the material's response fictitious. A disposition can be an objective feature of how something behaves in a specified interaction.

One proposed boundary therefore separates properties specified independently of a particular kind of perceiver from properties defined through perceptual responses. This classifies redness understood as a visual disposition differently from a spectral reflectance function. It likewise separates felt resistance from a specified mechanical hardness test. The boundary does not coincide with vision versus touch, or with measurable versus unmeasurable. Both colour-related and mechanical responses admit quantitative investigation; both can be relational.

There are limits to a stronger metaphysical interpretation. George Berkeley argues that perceived size, shape and motion also vary with perceptual circumstances; experience does not simply present an invariant class of primary qualities while presenting only relative secondary qualities. That challenges an easy appeal to resemblance. It does not, without further argument, show that there is no perceiver-independent structure, since variability of appearances is compatible with stable relations explaining the variation.^[ref]

The defensible conclusion is consequently qualified. Experienced hardness has no privileged immunity from the difficulties raised by redness. A distinction between perceptual responses and the physical organization explaining them remains useful, but its physical side must be established by inquiry. It is not an unrevisable list certified by whichever sense first reveals a property.

A distinction between physical constitution and perceptual response is plausible and scientifically useful. A division into wholly objective primary qualities and wholly unreal secondary qualities is much harder to defend. These are different interpretations of the primary and secondary qualities distinction.

John Locke attributes extension, solidity, shape, number and motion to bodies as primary qualities. Secondary qualities are powers to produce sensations through the constitution of bodies: a body's power to produce a colour experience is distinct from the experience itself. This distinction need not make the power unreal. Nor should the further resemblance thesis, that ideas of primary qualities resemble their causes, be confused with the more modest claim that bodily constitution explains experience.^[ref]

A modern model of colour perception shows what can be retained. For a nonfluorescent surface in fixed viewing geometry, a simplified cone cell response isHere $L$ is the incident spectral irradiance, $R$ the spectral reflectance, and $S_i$ an effective sensitivity for one of three cone cell classes. Neural processing and adaptation also matter, so these responses are not a complete identification of perceived colour. Nevertheless, the model separates the surface, illumination and observer rather than assigning every experienced variation to a changing surface.

It follows that one surface can produce different responses under different illumination. Conversely, distinct spectral reflectance functions can produce the same three responses for a specified illuminant and observer, a case of metamerism. Three weighted integrals do not in general uniquely determine a whole function. Thus identifying experienced colour with one intrinsic surface property loses both the dependence on the observer and the many-to-one character of the response.

The scientific benefits are concrete. A comparison of paints should specify illumination and observer conditions. A measurement intended to characterize a material can instead estimate its spectral reflectance, preserving information that a single colour match discards. In both cases the distinction tells an investigator what to hold fixed and what to vary. It organizes experiments and explains disagreement without requiring that one observer be hallucinating.

However, perceptual variability alone does not distinguish primary qualities from secondary qualities. George Berkeley points out that apparent extension and shape also change with viewpoint and perceiver.^[ref] A circular disc yields different projected shapes at different viewing angles. The stable object is inferred through a model of that variation; it is not simply copied into experience. Primary qualities therefore have no automatic exemption from the interpretative work involved in perception.

Nor is relationality equivalent to subjectivity. In special relativity, measured length depends on an inertial frame, but relations between such measurements are tightly constrained. Conversely, a disposition to affect a specified perceiver under specified conditions can be an objective physical fact. Classifying a property as relational does not settle whether it is real, measurable or explanatory.

The best distinction is consequently functional and revisable: distinguish the experience to be explained from physical properties and relations used to explain it. That retains the scientific benefits of separating constitution, environment and response. It does not establish an immutable list of primary qualities, a simple resemblance relation between experience and object, or a proof of scientific realism.

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.