As part of his philosophical writings, John Locke maintained that primary qualities and secondary qualities should be understood in different ways. The next two posts will consider whether Locke’s theory remains persuasive. In particular, I will refer to the contrary views of George Berkeley. Even if we reject the views of both Locke and Berkeley, it might be possible to reach an alternative interpretation of qualities.
Firstly, a ‘quality’ is commonly defined as a distinct property of an object that causes it to behave in a certain way. A beach ball has the property of ‘roundness’ while a piece of grass has the property ‘greenness’. The question arises in philosophy as to the source of these properties. Are they inherent in the object itself, or are they devised by means outside of the object, perhaps via the perception of an observer? Locke believed that a single definitive answer could not be supplied to adequately explain all properties, which led him to follow the approach of Robert Boyle in proposing two separate types of qualities.
A primary quality concerns a property of the object relating to its physicality in space. Typical primary qualities are size and shape (or ‘extension’ as it is often called). Additionally, Locke included ‘number’ as another primary quality. By ‘number’, Locke is not referring to a count of objects in an everyday sense, but rather a count of elementary particles.
It is worth briefly examining the idea of the elementary particles as these form an important part of Locke’s theories. Following the example of Boyle, Locke recognises that there is a single elementary particle which forms any number of combinations to create all the objects we see around us. However, these particles are not static, rather they are constantly in motion. This part of Locke’s theory will become more relevant once we consider secondary qualities in more detail.
A secondary quality is a property that doesn’t physically exist in the object. An observer needs to be present to perceive the quality. More specifically, the observer will use their own sensory powers to detect the quality, which then translates into a distinct impression upon the mind. Typical examples of secondary qualities are taste, smell and colour.
Locke’s definition of secondary qualities therefore stresses the presence of an observer in order to detect the quality. However, Locke is not saying that the secondary quality exists onlyin the mind of the observer. He is well aware that snow would still be white even if nobody was around to observe it. Therefore, it would be incorrect for us to say that the secondary object doesn’t exist at all if nobody happened to be present to perceive it.
If secondary objects do not exist entirely in the mind then, what is their source? Locke suggests that the object contains a type of ‘power’ to produce the secondary quality. This power comes about the unique motion of the elementary particles. Depending on what combination is produced, this disposes the object to act in a certain way when an observer is present. In fact, a modern philosopher would describe such a power as a ‘disposition’.
So to summarise Locke’s position in regards to secondary qualities, he recognises (1) the object must be disposed to act in a certain way (to produce a certain smell or colour), and (2) an observer must be present to detect the quality and translate the sensation directly into an idea in the mind. A primary quality, on the other hand, does not require the presence of an observer since the quality is an inherent, inseparable part of the object.