Primary and Secondary Qualities

by Keith Wilson

 

 

Introduction

Galileo, Descartes, Hobbes and Locke all make the distinction between primary qualities , i.e. those that are essential to and inherent in a thing, and secondary qualities , which include sensory impressions such as colour, taste, smell, heat, and so on. Although they each present different arguments for this distinction, their explanations point towards a picture of reality that differs radically from that of everyday experience. This essay considers to what extent these arguments contribute to an objective understanding, both of the nature of reality and the process of perception, before finally turning to Locke’s notion of a Power and its implications for these two issues.

The Nature of Reality

Galileo’s first argument for the existence of primary qualities 1 is a purely rational one. He asks himself what properties must a thing have in order to exist? and concludes that only extension , shape , size , position (in space, time and relative to other bodies), motion and number are required. Removing any one of these qualities would effectively cause the object to cease to exist, and so he concludes that they must be the primary (i.e. essential) properties of physical matter.

Other properties, however, such as an object’s colour, sound, taste or smell may be altered or removed without affecting its physical existence, and these he concludes must be secondary or ‘accidental’. An ice cream, for example, does not cease to be such regardless of whether it is blue, green or chocolate chip flavoured. However, an ice cream with no shape or extension is both logically and physically impossible, and so cannot be classed as a material thing. In a similar way, Galileo reasoned that changing the colour, appearance or even shape of a thing does not affect its essential thing ness (i.e. its existence as a material object) in the way that removing any of its primary properties does.

The problem with this approach is that it relies solely upon our ability (or inability) to imagine various combinations of properties, which in turn is based upon our experience and therefore evidence gained through our senses, which may be inadequate to the task at hand. To reach an impartial account of reality, it is first necessary to determine both where such qualities reside and how they may be described objectively.

The question of where properties reside is tackled in several ways. Galileo likens the experience of taste, smell and colour to the sensation of being tickled, which “belongs to us and not to the hand” or other object doing the tickling. 2 Similarly, Descartes and Locke both cite the example of pain, a subjective phenomenon not considered to be present in the object causing the pain, as a model for other perceived qualities. 3 Descartes’ reflections on the power of writing to evoke “thoughts and emotions”, 4 Hobbes’ observation that the image of an object can be separated from its source, 5 and Locke’s that objects have no colour in the absence of light 6 show that appearance can in fact be separated from substance. Similarly, pressing on one’s eyes or ears can create the impression of light or sound even though these properties are not present in the external object that causes them, 7 all of which supports the premise that such qualities are purely subjective and exist only in the mind.

However, it is the thesis that secondary qualities are nothing but manifestations of a thing’s primary qualities that provides the strongest grounds for the distinction between the two categories. According to this view, 8 each of our senses is capable either of perceiving an object’s primary qualities directly or some aspect of its primary qualities (shape, size, motion, etc.) that is otherwise hidden to us, but nevertheless gives rise to the secondary qualities we experience (e.g. smell, colour and taste). 9 This establishes a clear hierarchy of qualities by which the nature of the material universe is shown to be more fundamental or ‘real’ than our subjective experiences of it. 10 Thus, the physical world exists without colour, smell, taste, warmth or sound, and may be described solely and objectively in terms of its primary qualities. Such secondary qualities exist only in our minds as the result of sensory interpretation of the primary qualities of matter, and therefore in no way resemble the objects that we normally attribute them to. 11

The Process of Perception

In order to prove this thesis, it is first necessary to prove a causal link between our ‘subjective’ (i.e. secondary) perceptions and the ‘objective’ (i.e. primary) qualities of physical matter. This is explained by Locke in terms of “insensible particles” that are themselves too minute to be observed directly, but whose shape, size, motion etc. give rise to our sensory impressions. 12 Here, Locke follows Descartes and Hobbes in likening all forms of perception to the sense of touch, which perceives the qualities of size, extension and motion directly. In each case, the object is said to press upon 13 or cause “local motions” 14 (i.e. internal changes of state) in the relevant sensory organ, with insensible particles creating a chain of cause and effect that bridges the apparent distance between the object being observed and the observer. 15 The end result of this chain is the production of an idea in our mind corresponding to, for example, the colour blue.

Although modern physics and biology confirm the existence of such particles (sight is mediated by photons, sound by vibration of particles, taste and smell by the shape and size of molecules), the texts differ about the precise details of this process. Hobbes argues that all of the senses functions by means of actual physical pressure, 16 whilst Descartes and Locke cite the more general concepts of motion and impulse , respectively. 17 The mechanism of nerves in the body is also invoked to explain how such phenomena are translated into subjective experience, 18 but the precise workings of these is (with the notable exception of Locke, discussed below) left a mystery.

For Descartes, one important consequence of this line of reasoning is that it redefines the very concept of a ‘thing’. 19 In contrast to the previously accepted doctrines of Aristotle and scholastic philosophy, 20 the “substantial form” of a thing cannot be said to reside in the material object, except in terms of a particular configuration of its primary qualities. For example, there is nothing inherent to a tennis ball that resembles our perception of ‘a ball’ any more than it contains anything resembling ‘tennis’. For Descartes, the ball is simply an idea that exists in our minds, derived from a combination of sensations that depend on the primary qualities of physical matter. 21 Reality therefore consists not of the conventional forms we perceive and experience every day, but is simply ‘matter in motion’, all of which shares the same fundamental nature and primary qualities.

Powers and Interactions

In addition to the distinction between primary and secondary qualities, Locke proposes a third category: the Power of one thing to interact with or alter another. 22 Having established the derivative nature of secondary qualities, he goes on to compare the Power of a thing to produce sensations to the Power of a hot object (e.g. the sun) to change the colour or consistency of wax. 23 The fact that these processes are seen as equivalent is an important step towards understanding the physical process of perception. Not only do secondary qualities result from an object’s primary qualities, but the object causing these sensations is said to “alter the Bulk, Figure, Texture, or Motion of some of the insensible parts of my Eyes, or Hands, as thereby to produce in me the Idea of Light or Heat.” 24 In other words, perception itself is the result of altering certain primary qualities of the observer’s senses . Although Locke stops short of saying that thoughts and ideas are themselves the result of primary qualities in the observer’s brain, this is a natural extension of his line of reasoning and offers a physical basis for all thoughts and sensations that arise in the mind.

Superficially, Locke’s conception of a Power suggests a return to the Aristotelian notion that qualities are inherent in things and so exist externally to the mind. However, as he is at pains to point out, such Powers exist only as a consequence of a thing’s primary qualities 25 and as such, are themselves secondary to their ability to produce a certain effects either in us or in other observable objects.

Conclusion

The above arguments demonstrate that the primary qualities of matter do not include perceived qualities such as colour, taste, sound, smell or heat, which exist only as ideas or sensations within our minds, and are not inherent to the objects that we perceive. Modern science confirms the existence of “insensible particles” that mediate perception and a nervous system that translates their primary qualities into electrical impulses within our brains. However, the picture of the external world being entirely independent of our experience of it is not entirely correct.

Firstly, since these theories are based upon experience, we can say nothing about what the world might be like without our being there to observe it. Secondly, the physical basis for all secondary qualities does actually exist in the things we perceive . The critical difference being that this physical basis does not resemble our experience of it in the way that the qualities of extension or size appear to. However, whilst this proves that some properties are indeed more fundamental than others, we cannot be certain that primary qualities such as extension are not actually manifestations of other qualities, e.g. motion, 26 or of something even more fundamental that is otherwise unknown to us.

 

© 2004 Keith Wilson, University of York