Primary and Secondary Qualities

Although Locke assumed that simple ideas of sensation are invariably produced in our minds by the influence of external objects on our organs of sense, he did not suppose this causal process to be straightforwardly representational: there need not always be something in the object that corresponds directly with the idea it produces in us.

In some cases, for instance, it is the absence, rather than the presence, of some determinate feature in the thing that produces "a real positive Idea in the Understanding." [Essay II viii 1-2] Even on a causal theory of perception, privation is a change in the sensory organ, and therefore fully perceptible. Thus, our ideas of cold, darkness, and (perhaps) rest are produced by the absence of heat, light, and motion in things.

Despite the systematic ambiguity of our usual vocabulary, Locke maintained, we should always distinguish between ideas themselves, the immediate objects of thought as entertained by the mind, and qualities, the causal powers by means of which external objects produce those ideas in us. [Essay II viii 7-8] An examination of the causal processes involved in formation of our sensory ideas thusly led Locke to propose an important distinction among the qualities of bodies:

The primary qualities of any body are features Locke took to be inseparable from it, both in the sense that they are the kinds of features we experience to be aspects of every body and in the sense that we suppose that they would remain present even if the body were divided into imperceptible parts. [Essay II viii 9]

Our ideas of the primary qualities include those of solidity, extension, figure, motion, rest, and number. Notice that all of these ideas are perceived by touch and that all except solidity are perceivable by sight as well.

Since bodies operate only on mechanistic principles, our perception of such ideas can only result from a direct contact between our organs of sense and the object itself-or, in the case of vision at a distance, an imperceptible particle from the object itself. [Essay II viii 12] Such qualities therefore exist as features of the body itself, independently of their perception.

Secondary qualities, on the other hand, Locke held to be nothing in the object itself except a causal power to produce ideas of a certain sort-a color, a taste, or a sound, for example-in us. Like the mere powers of producing change in other objects, these supposed qualities are nothing other than effects produced by the genuine (primary) qualities of bodies themselves. [Essay II viii 10]

Notice that all of our ideas of secondary qualities are produced in us only by a single sense; their perception clearly depends upon the normal operation of our sensory apparatus in response to the merely dispositional properties of things. Thus, our customary supposition that such qualities really exist in nature is nothing more than a reification of the ideas that are actually produced in our minds by the primary qualities of things. [Essay II viii 14-17]

Locke's arguments in defence of this thesis may be familiar from their prior statement in Galileo, Descartes, and Boyle or from their later appropriation by Berkeley and Hume.

The color and taste of manna (a natural laxative) are less like its powdery texture than like its effect on the digestive tract; although things themselves remain the same in darkness as in light, their color appears only through the mediation of rays of light; contrapositively, the mechanical process of grinding an almond can change only its primary qualities,

yet the secondary qualities are thereby modified; and a single body of tepid water can seem both warm and cool at the same time, if our fingers have been differentially chilled and heated beforehand. [Essay II viii 18-21] In every case, the proper conclusion is that the ideas of secondary qualities are produced in us only by the mechanical operation of the primary qualities of bodies.

From this distinction, Locke concluded that there is a significant difference in the representational reliability of primary and secondary qualities. Our ideas of primary qualities resemble the qualties themselves; the mind-independent features of material objects are (at least at the microscopic level) exactly as we perceive them to be. But our ideas of secondary qualities and powers resemble nothing in the material world; they are the causal consequences, in our minds, of the mechanical action upon our sensory organs of corpuscular primary qualities. [Essay II viii 23-24]

As his Royal Society companion Robert Boyle had shown, the rapid motion of insensible particles of matter is a genuine feature of objects, a primary quality that can be transmitted mechanistically from one body to another: as a result, fire has the power to melt other bodies and-in the same way-to produce in us the ideas of warmth, heat, or even pain. [Essay II viii 15-16]

Ideas of Reflection

Although he insisted that human awareness begins with simple ideas of sensation, Locke did not believe that they comprise all of our mental contents. We also acquire simple ideas of reflection through an objective observation of our own mental operations. In any these operations, the human mind must be either passive or active, Locke supposed, and the most fundamental ideas of reflection are therefore just two in number:

perception, in which the mind passively receives ideas, and volition, in which it actively initiates something. [Essay II vi 1-2] Every other idea of reflection, on Locke's view, is a simple mode of one or the other of these two basic types. All human mental activities derive from the faculties of understanding and will.

We are naturally familiar with such activities, since they constitute the whole of our conscious thought, but this does not entail our having clear conceptions of them. Familiarity without careful attention provides only confused ideas of our own mental operations. As Locke had already noted, the components of experience we first acquire are vivid sensory impressions of the external world. It is only with increasing maturity and a capacity for detachment that we grow able to make the careful inward observations from which clear ideas of reflection may arise. [Essay II i 7-9]

Even when we have acquired clear ideas from reflection, Locke supposed, we often designate them with a vocabulary drawn from the concrete context of sensory experience, representing our "inner" mental operations by an implicit reference to observable "outer" processes. [Essay III i 5]

The first and simplest of our ideas of reflection is that of perception, the passive reception of ideas through the bodily impressions made by external objects upon the organs of sense. Although perception in this sense is distinguished from thinking generally by the relatively meager degree to which it falls under our voluntary control, Locke believed it always to require some degree of conscious attention.

Physical stimulation of the sensory organs does not produce ideas unless the mind is attentive. [Essay II ix 1-4] Here Locke tilts to a significant feature of our sensory experience. Although we are almost constantly bombarded by physical stimuli-any one of which will, in the ordinary course of things, produce in our minds a particular sensory idea-we never perceive all of them and sometimes exercise deliberate control over which ones we do perceive.

Attention, choice, and judgment can all influence the operation of the officially passive faculty of perception. [Essay II ix 7-8] Another of our cognitive capacities is the ability to retain ideas in the mind over time, either in continuous contemplation or in recollection of ideas after a period of inattention. Although he naturally relied upon the spatial metaphor of memory as a "Store-house" or "Repository" of ideas, Locke emphasized that the ideas we can recall are not actually in the mind-that is, we are not conscious of them at all-during the inattentive interval. Memory is just the power to bring to mind ideas that we are aware of having perceived before without simply perceiving them anew. [Essay II x 1-2]

Like Descartes, Locke assumed that the proper function of human memory relies upon some physical process within the human body, as shown by the gradual effacement of infrequently experienced ideas and the loss of memory as a result of bodily disease. [Essay II x 4-5] On the other hand, some ideas are more firmly and easily retained than are others.

Greater degrees of attention and frequent repetition of experience have some influence on this, but Locke supposed that the most important factor is the association of certain of our ideas with pleasure and pain. The retention of ideas thusly correlated with the achievement or loss of happiness is one of the benevolent provisions for the needs of human life, since it motivates us to avoid or prefer experiences of appropriate sorts. [Essay II x 3]

Being able to remember our past experiences of hunger or headache without actually feeling their pain, he argued, encourages us to act in such ways as are likely to forestall their recurrence. [Essay IV xi 5-6]

Our reflective ideas of other mental operations receive much shorter treatment. Discerning is the mental operation of distinguishing among our ideas--either quickly by wit or carefully in sober judgment. This is a vital tool in providing the supply of distinct ideas whose interrelations we intuitively grasp in assenting to self-evident truths. [Essay II xi 1-3]

In the same fashion, the activity of comparing gives rise to our knowledge of relations by noticing non-exact similarities among our ideas. [Essay II xi 4-5] Compounding is the mental capacity to manufacture new complex ideas out of relatively simple components, while abstracting is the ability to make general use of particular ideas in more general reference by stripping away their indexical features.

These processes are especially vital because they ground our linguistic competence in employing general terms and secure the possibility of universal knowledge. [Essay II xi 6-9] Distinguished by their phenomenal quality, other cognitive powers are comprehended by the reflective ideas of sensation, remembrance, recollection, contemplation, reverie, attention, study, dreaming, and ecstasy. [Essay II xix 1-2]

Animal Thinking

Although each human being must acquire the ideas of reflection by observation of her own mental operations, on Locke's view, the activities thusly conceived may then be attributed to other sentient beings, qualified with the suspicion that they may vary greatly in degree from case to case. [Essay III vi 11]

There is ample evidence of individual variations even among similarly-constituted fellow humans, and it is reasonable to suppose that non-human thinkers could differ in ways that we cannot literally imagine. Although our own faculty of perception is perfectly suited to the conduct of human life, for example, Locke noted explicitly that other spirits, whose embodiment involves sensory organs different from our own, might well perceive aspects of the world of which we are unaware. [Essay II xxiii 12-13]

Dependent as we are on a sense of sight with a particular acuity, we can only speculate how the natural world might appear to beings whose visual abilities were different, much less those for whom the senses of smell or hearing are more fundamental.

Although Descartes and his followers had dogmatically argued that non-human animals, lacking possession of a separable thinking substance or soul, must therefore be incapable of thinking in any form, Locke preferred to examine the empirical evidence in an effort to discover to what degree animals may be capable of engaging in mental activity of each sort. [Essay II i 19] The intellecual capacity for perception, Locke held, marks the distinction between animal and plant life, and the sensory faculties of each species are well-suited to the practical needs of life for animals of that sort.

Since individual human beings differ widely in their intellectual abilities, Locke even speculated that those of some animals may exceed those of the least gifted of human thinkers. [Essay II ix 11-15] In the case of the retention of ideas, he suggested that the behavior of songbirds clearly exhibits their deliberate effort to learn a new tune. [Essay II x 10]

Nevertheless, Locke believed that the higher intellectual abilities of animals are strictly limited. They do not compare or discern ideas that lie beyond their immediate "sensible Circumstances," nor are they capable of compounding the new complex ideas required for engaging in mathematical thought. [Essay II xi 5-7] Most notably, he maintained that non-human animals are utterly incapable of abstraction.

Even if their sensory capacities permit them to engage in some degrees of intuitive thinking, then, their inability to form abstract ideas and employ general terms make it impossible for them to rise above particularity in order to engage in moral reasoning.

[Essay II xi 10-11] Thus, Locke preserved a significant moral distinction between human beings and other animals without grounding it on an ontological distinction in the possession of an immaterial soul. ©1999-2002 Garth Kemerling.Last modified 27 October 2001.Questions, comments, and suggestions may be sent to: the Contact Page.