Having defective organs of sense...

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.