Past exam of the philosophy of science supplementary subject of the University of Oxford / 2015 / 1 / Solution

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.