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.
Origin in Experience
Since knowledge-indeed, human thought of any sort-is 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 beings-having, for all we know, experiences very different from our own-are 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]
Ideas of Sensation
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.