19th January 1999

(1,650 words)

What is Locke's distinction between primary and secondary
qualities? Is it tenable?

Whilst acknowledging criticisms, such as those of Berkeley, I wish to suggest that Locke's distinction between primary and secondary is largely tenable. By distinguishing between those qualities of bodies which are independent of human consciousness and senses of individuals, important distinctions are made about the world and of what we can claim to have true knowledge of. The main distinctions that I wish to highlight between primary and secondary qualities are firstly, that primary qualities are linked to the essence of a substance and are therefore 'inseparable' from it, strictly obeying scientific laws. The second major distinction is that individual's ideas of primary qualities have greater resemblance to reality than do their ideas of the secondary qualities, which do not necessarily resemble anything as they are conceivable not to exist in the absence of an observer.

Locke, writing after Descartes' meditations, and influenced greatly by the scientific writings of Newton and Boyle subscribed to an atomisitic view of the world. His conception was of substances made up of tiny particles, the movement and structure of their interactions giving the substance its form and properties. Certain things will always be true of these substances, such as that they have specific mass, size, number and motion. Nothing can prevent the substance from exhibiting these qualities which Locke describes as the 'primary qualities'. Included also in Locke's account of primary qualities is that of solidity. This is related to the inability for two substances to occupy the same space as each other. This notion must therefore be contrasted with the secondary qualities, such as temperature, colour, sound, smell and taste. These 'secondary' qualities are all related to human sensation and detection of certain stimuli. This is what Locke describes as the 'power' to produce motion in an individual's 'animal spirits'. The animal spirits refers to the model of the time of how the human nervous system works. This model suggests that stimuli will act upon these 'animal spirits' through the sensory organs which move around the bodies nerves until they are interpreted by the mind as sensations. According to Locke, the secondary qualities are 'nothing more' than the ability of a substance to cause such sensations.

To move Locke's model slightly further, the secondary qualities are dependent on the primary qualities of the substance. For example, it is the texture and density of a substance that will effect what spectrum of light is reflected and absorbed by the substance and so its colour. There is nothing of the colour in the substance, but instead reactions to forces and other substances, which when viewed will give an individual the sensation of colour. From this it can be inferred that whilst in the absence of an observer a substance will be expected to exhibit all its primary qualities, it will have no secondary qualities. A primary quality is therefore non-relational, not dependent on other substances.

The second important feature of the Lockean model is that of resemblance. Locke states, 'the ideas of primary qualities of bodies are resemblances of them, and their patterns do really exist in the bodies themselves... but there is nothing like our [secondary quality] ideas existing in the bodies themselves'. There is an analogy that can be drawn here. It is conceivable for two people to see one colour and describe it as red, yet have two completely different sensations in their mind, which, because they are never contradicted through their life means that two people could both gain different experiences from the same stimuli, yet label it as the same. It is inconceivable for the same to be said of primary qualities, such as shape. Placing a sheet of paper flat on a table could never be confused with placing a sphere on the same table. However detected, the points of contact would be different, so it would be impossible for two people to construct different models in their minds of the nature of the objects as any movement of the objects would show differences and so make a false model easy to detect. Whilst it is possible to be colour-blind, it is not conceivable that two people could have a different shapes in their minds that they would be deceived as to be thinking that they are the same thing.

Another perspective to the resemblance theory is the different ways that secondary qualities can be expressed. It is for example suggested that bats, can distinguish between objects that we see as being different colours by use of sound. What a bat might detect as colour we might call pitch, yet they are representing supposedly the same thing. Thermal imaging cameras also represent heat, not as the sensation that we would associate with temperature, but instead as colours. If our sensory organs were formed using the same mechanisms as such devices, would we not be claiming that there is something in warm substances which would have resemblance to colour.

This difference could suggest that primary qualities might be able to be accurately be formed a priori. For example, if the mind is to create the idea of a shape with four equally length sides at right angles, a square would appear that would be the same as a substance that would be described as a square. They would both obey laws of geometry. If the idea fails to exhibit the same geometry as the reality, the idea is wrong, it can not be subjective. The same is not true of the secondary senses. How can someone be deemed 'wrong' for having a sensation of a sounds as being higher pitched than other believe it is, when pitch is simply the detection of variation in wavelengths as the air is vibrated. It is possible for half the world to have an inverted sense of pitch, where it is impossible for half the world to see a square as containing curved edges or five degree angles.

Another perspective to this is by looking at what can change the idea of a secondary property. By numbing the taste buds it is easily possible for the same individual to taste a lemon as being less bitter, yet the lemon has not changed in any way. For this reason, it is clear that it could not be claimed that the idea of the secondary qualities would resemble anything in a substance when its relational properties mean that changes in other substances will easily affect how it is perceived. The criticism of Locke's resemblance model is more likely to come from suggesting that the notion that primary qualities resemble their ideas is as equally false. But I believe the a priori laws of the action of primary qualities suggests that the resemblance must be utterly consistent and so fitting to be described as a 'resemblance'.

Berkeley's central criticism to Locke's model is that he believes it is inconceivable to have an idea of substances without exhibiting the secondary qualities. He states, 'It is not in my power to frame an idea of a body extended and moved, but I must withal give it some colour or sensible quality'. This does not seem to dismiss Locke's distinction. When Berkeley looks at a substance he will be sensing both the primary qualities and the 'power' that the secondary qualities will have over his 'animal spirits' (to maintain Locke's terminology). This will mean that he will always associate the primary and secondary qualities as being part of the substance. The question still remains as to whether it is possible to imagine any difference that would occur to the secondary qualities that could not be linked directly to the primary qualities. A vibrating rubber band cannot create a different pitch unless there is action against the motion, length or other primary qualities of the band. For there to not be a distinction between primary and secondary qualities, surely changes in these secondary qualities must be independent of their primary qualities, if not, Locke is correct in raising the distinction.

An interesting criticism of the resemblance model is known as 'Molyneux's Question'. This has so far been impossible to prove empirically. A blind man is presented with a cube and sphere to touch and so form an idea of them. If he then regained his sight, would he be able, by use of other senses be able to name which is which? According to Locke, surely the man's idea should be a resemblance of the primary qualities and so the addition of sight would re-enforce this idea and not confuse it. There does seem great difficulty in this example, but still, I believe that if the primary qualities and the geometric properties of the objects are explained to the blind person, the model that they would be forced to produce in their mind, re-enforced by the feel of the substance would give a consistent model. When exposed to the sight of the object the resemblance might not be obvious, but when investigated would be noted to be consistent. The uncertainty of this question, does not, however, make Locke's distinction between the primary and secondary qualities untenable.

In all, Locke does highlight some important distinctions between primary and secondary qualities. The differences both in their inseparability from the substance and the resemblance to ideas do hold and so making his system as useful classification. The criticism of Berkeley seems to be too dependent on reliance on the senses and believing the senses. The scientific laws which seem able to be understood in an a priori way can be seemingly applied to primary qualities that cannot be to secondary qualities, which are dependent on changes in primary qualities to change. There does seem to be a tenable distinction. When a tree falls in a wood and there is no one to hear it, it doesn't make a sound.