Locke used the word "idea" for the most basic unit of human thought, subsuming under this term every kind of mental content from concrete sensory impressions to abstract intellectual concepts. Explicitly disavowing the technical terms employed by other philosophical traditions, he preferred simply to define the idea as "whatsover is the Object of the Understanding when a Man thinks." [Essay I i 8] Locke worried little about the ontological status of ideas. He did commonly refer to them as being "in the Mind," both when we are conscious of them and when they are stored in memory, he regarded this as no more than a spatial metaphor. Locke was interested in these immediate objects of perception only because they point beyond themselves.
Thus, the crucial feature of ideas for Locke was not what they are but rather what they do, and the epistemic function of an idea is to represent something else.
For since the Things, the Mind contemplates, are none of them, besides it self, present to the Understanding, 'tis necessary that something else, as a Sign or Representation of the thing it considers, should be present to it: And these are Ideas. [Essay IV xxi 4]Because we do think and must always be thinking about something or other, then, it follows that we actually do possess ideas. [Essay II i 1] If we want to comprehend the foundations for human knowledge, Locke supposed, it is natural to begin by investigating the origins of its content.
Since knowledgeindeed, human thought of any sortis mediated by ideas, it is well worth asking how we acquire them. Thus, in Book II of the Essay, Locke embarked on an extended effort to show where we get all of the ideas that we do so obviously possess. An adequate genetic account will explain, at least in principle, how human beings acquire the ability to think about anything and everything.
Let us then suppose the Mind to be, as we say, white Paper, void of all Characters, without any Ideas; How comes it to be furnished? Whence comes it by that vast store, which the busy and boundless Fancy of Man has painted on it, with an almost endless variety? Whence has it all the materials of Reason and Knowledge? To this I answer, in one word, From Experience: In that, all our Knowledge is founded; and from that it ultimately derives it self. [Essay II i 2]The human mind is like a camera obscura for Locke, a darkened room into which bright pictures of what lies outside must be conveyed. [Essay II xi 17]
Locke had already argued at length that ideas are not innately imprinted on the human mind. Observing children reveals that their capacity to think develops only gradually, as its necessary components are acquired one by one. No individual idea is invariably present in every human being, as one would expect of an innate feature of human nature, and even if there were such cases, they could result from a universally-shared experience. Everything that occurs to us either arrives directly through experience, or is remembered from some previous experience, or has been manufactured from the raw materials provided solely by experience. [Essay I iv]
From the outset of the project, then, Locke took the empiricist stance that the content of all human knowledge is ultimately derived from experience. We can only think about things we're acquainted with in one or the other of two distinct ways:
Our Observation employ'd either about external, sensible Objects; or about the internal Operations of our Minds, perceived and reflected on by our selves, is that, which supplies our Understandings with all the materials of thinking. These two are the Fountains of Knowledge, from whence all the Ideas we have, or can naturally have, do spring. [Essay II i 2]Notice that Locke distinguished sensation and reflection by reference to their objects. We acquire ideas of sensation through the causal operation of external objects on our sensory organs, and ideas of reflection through the "internal Sense" that is awareness of our own intellectual operations. As the rest of Book II is designed to show, these two sources provide us with all of the ideas we can ever have. [Essay II i 3-5]
The acquisition of ideas is a gradual process, of course. Newborn infants, Locke supposed, are first aware of the vivid experiences of their own hunger or pain. Then, by further experience, they acquire a supply of sensory ideas from which they can abstract, learning to distinguish among familiar things. Only later do they attend to their reflective experience of mental operations in order to acquire ideas of reflection. [Essay II i 21-24] Since we come to have ideas only by means of our own experience, Locke supposed, any interruption of this normal process could prevent us from having them. Having defective organs of sense, artificially restricting experience, or inattentively observing what we have can all limit our possession of mental contents. [Essay II i 5-8] Individual human beings therefore exhibit great differences in their possession of simple ideas, and Locke speculated that other sentient beingshaving, for all we know, experiences very different from our ownare likely to form ideas of which we can have no notion at all. Since simple ideas are acquired only by experience, anything we do not experience is literally inconceivable to us. [Essay II ii 1-3]
Everything begins, then, with simple ideas of sensation. Most of these are uniquely produced in the mind through the normal operation of just one of the organs of sense. Our ideas of colors, sounds, smells, tastes, and heat, Locke supposed, are acquired respectively through our eyes, ears, noses, tongues, and skin. Lacking the appropriate organ in any of these cases would wholly prevent our having any of the characteristic ideas of that sense. With normal sensory organs, we come to have so many simple ideas of sensation that we don't bother to invent words naming all of them. [Essay II iii 1-2] Notice that these ideas tend to be either the unnamed determinate instances of some determinable predicate (particular shades of blue and varieties of sourness) or sensations easily identifiable by association with other ideas (the taste of pineapple and the fragrance of a rose).
According to Locke, certain special simple ideas are acquired by two different senses. Space, extension, figure, motion, and rest are all presented to us both in sight and in touch; they are therefore among the most commonly received of all our ideas of sensation. [Essay II v] Because of their prominence, specific ideas of these kinds constitute the basis for the most fundamental organization of our sensory experience. All of them represent primary qualities of sensible objects and serve significant roles in science and ordinary life. Things that can be both seen and touched seem most obviously real to us.
But is it correct to suppose that one and the same idea can be acquired from either of two distinct senses? Since simple ideas of sensation cannot be acquired through defective sensory organs, on Locke's view, it should be impossible to acquire the visual idea of motion from tactile sources alone. Locke's Irish friend William Molyneux posed this problem with precision in letters to the Bibliothèque Universelle and to Locke himself. [Correspondence 1064, 1609, 1620.] Supposing that someone blind from birth became familiar with solid figures by touch alone and then later gained the power to see, would this person be able to distinguish a cube and a sphere by sight alone without first touching them? In a passage added to the Essay's second edition, Locke agreed with Molyneux (on a view confirmed by twentieth-century empirical research) that the answer must be no. [Essay II ix 8] The visual and tactile ideas of the globe are distinct. Although we use the same words to designate ideas of shape and motion whether they have come from sight or from touch, only "an habitual custom" associates ideas from distinct senses with each other.
In a few, even more special instances, simple ideas are produced in us by reflection as well as sensation. These are ideas that are invariably present in the mind in association with every other object of thought, no matter what its source. According to Locke, such ideas of both sensation and reflection include pleasure, pain, power, existence, and unity. [Essay II vii] Among these existence and unity secure yet another aspect of the organization of our experiences, by providing clear conceptions of reality. Pleasure and pain, as we'll see later, play a special role in motivating us to exercise the volitional power behind all human actions, of mind and body.
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 itselfor, 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 sorta color, a taste, or a sound, for examplein 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 andin the same wayto produce in us the ideas of warmth, heat, or even pain. [Essay II viii 15-16]
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 stimuliany one of which will, in the ordinary course of things, produce in our minds a particular sensory ideawe 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 mindthat is, we are not conscious of them at allduring 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]
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.