Robert Boyle’s essay ‘Origin of Forms and Qualities’ starts with the following principle: all bodies consist of the same universal (‘catholic’) matter. Because bodies vary quite significantly in their appearance, there must be a further way to explain the diversity in the bodies around us. Boyle concludes that motion is responsible for causing these ‘various tendencies’.
The combination of matter and motion therefore gives rise to primary qualities of size and shape. These are inherent properties that exist in all bodies of finite size, no matter how large or small. However in addition to the primary qualities, Boyle notes that we also have properties that are detected by sensible and rational beings, such as sound, smell and colour. Because these properties have been apparent to us since we were born, we take it for granted that the ‘sensible qualities’ are real properties of the objects in question. However, Boyle believes that this view is too simplistic. When we look at bodies, all we find are the physical attributes of size and shape. The sensible qualities do not actually ‘exist’ in any tangible sense. Rather, the sensible qualities are formed by the mind.
Boyle also identifies pain as another example of a secondary quality. When we see a pin, there is no inherent property of pain visible in the pin. It is only with a certain contact with the pin do we feel the sensation of pain. So human experience must be a combination of primary qualities and ‘experienced qualities’. In the case of the latter, these don’t exist in the objects themselves, but are formed in the mind (which Boyle refers to as the ‘soul’). Therefore, Boyle sets up a system similar to Descartes in that there is a distinct division between body and soul.
One objection to this ‘corpuscular hypothesis’ is that the secondary properties would still exist even if no humans were around to experience them. Snow would still be white and burning coal would still melt wax even if nobody was around. So this suggests that the secondary qualities aren’t just formulations of the mind and do actually exist in the outside world.
Boyle next deals with this objection, and comes to the conclusion that secondary qualities do indeed exist in two senses. His argument proceeds as follows:
- Sensible qualities actually depend on physical qualities
- We can determine a thing’s secondary qualities without having to directly experience the thing in question. We can apply our knowledge to determine how something should appear or act
- Primary qualities aren’t just the obvious physical attributes such as size and shape but also include the effects of all the tiny particles in motion
- It is the tiny particles in motion that create the disposition for the secondary qualities, which allows an animal to detect the quality via its sensory organs.
As an example of this proposition, Boyle mentions an out-of-tune lute. Even if the lute isn’t being played, it still has the property of being out-of-tune. Nobody has to hear the lute being played to detect that the property exists. Therefore, this is an example of a secondary quality that exists within the object itself, even if we can’t actually see it.
Boyle notes this is similar to how snow is always white even if nobody is around to observe the snow. If there were no animals in the world, the snow might not actually LOOK white, but importantly, it would still have the disposition to appear white for anyone who happened to observe it. So a secondary quality is a type of disposition in the object to produce a certain kind of experience.
In terms of the ‘whiteness’ of snow, we now understand that this is just the result of matter reflecting / absorbing certain frequencies in the electro-magnetic spectrum. This property would still exist even if nobody was around to observe it, so Boyle is correct to note that secondary qualities do arise directly from the object and are not inferences of the mind. Humans just happen to see the colour white when they look at snow, but if another intelligent being were to come along, they might perceive the snow in a different way. So human perception does form an important part of how we perceive secondary qualities.
So overall, in explaining what secondary qualities are, a philosopher could claim that they are (a) completely subjective in the minds of humans (the qualities would not exist if humans did not exist) or (b) completely objective in the objects themselves (they do not require an observer to perceive the qualities). Boyle takes a middle ground approach by suggesting that the disposition to produce a certain quality is objective, but the actual quality itself requires a subjective point-of-view.
We could therefore conclude that secondary qualities are best understood by thinking about how humans experience them, as well as recognising that a secondary quality may not necessarily depend on human observation alone. In other words, the secondary quality contains both a subjective and an objective element.