Locke's purpose in the Essay is "to enquire into the origin[al], certainty, and extent of human knowledge; together with the grounds and degrees of belief" (I.I.2). He calls himself an 'under-laborer' for the work of Newton, Boyle, and others -- including the physician Sydenham. (Locke himself studied medicine, and served as a physician at times.)
Locke's 'way of ideas': all our knowledge is 'built out of' ideas. The only things our minds directly 'encounter' or 'grasp' or 'contain' are ideas, so our knowledge can go no further than our ideas and (our perception of) relations between them. (In Locke's words: "Since the mind, in all its thoughts and reasonings, hath no other immediate objects but its own ideas, ... it is evident that our knowledge is only conversant about them" (IV.I.1).) Locke also says that the meaning of a given word is just the idea associated with that word in the mind of the speaker.
Locke's "historical, plain method": examine the ideas we do in fact have, and examine how we came to have them (that's the 'historical' part). For example, in parts of the book we do not read, Locke argues that all our ideas have their ultimate source in sensation: our sensations provide us with particular fundamental ideas, and then our minds combine or otherwise process these fundamental perceptual ideas to create new ones.
Locke: solidity is the essence of body. When we examine our idea of body, Locke claims, we find that if something is a body, then that thing is solid, and conversely (i.e., all solid things are bodies). In Locke's words: solidity is "seems the idea most intimately connected with, and essential to, body." What does Locke mean by 'solidity'? A: "That which hinders the approach of two bodies, when they are moved one towards another, I call solidity." As we shall see later, solidity is not the only thing Locke considers "inseparably inherent" in physical bodies.
Further remarks on solidity and body.
- "Solidity fills space": Solidity is what prevents 2 bodies from occupying the same space at the same time. [Imagine two overlapping circles: if bodies were not solid, material bodies could do that.] That is, solidity accounts for the impenetrability of body.
- Contra Descartes: Descartes' identifying the essence of body as extension is untenable, since it does not capture the impenetrability of bodies.
- Solidity is not the same as hardness: a water droplet is soft, but just as solid as an iron frying pan, since the water repels any other body from occupying its space. The difference between water and iron is in the strength of cohesion among the particles in the droplet and in the pan. Locke recounts an experiment to show that water resists other bodies: a metal ball was filled with water and then sealed; then the ball was squeezed; the ball appeared to 'sweat' drops of water. Locke takes this to show that water, just as much as metal, is solid and impenetrable.
- All mechanical philosophers agree that bodies can communicate motion to one another via impulse. But how is the motion passed from one to the other? Locke says it depends upon the solidity of the bodies in the interaction: if there were not a repulsive force in the bodies preventing them from sharing the same space, motion could not be transmitted via impulse from one to the other in the manner in which it actually happens. (II.IV.5)
- But what is solidity, really? Locke's answer to anyone who asks this question is: "I send him to his senses to inform him" (II.IV.6). The notion of solidity is a simple idea drawn from perceptual experience -- if we try to explain in words what these fundamental sensory ideas really are, Locke says, it is like trying to explain what seeing red is really like to a blind person.
How do bodies (and their qualities) produce ideas in us? Locke's answer: "by impulse, the only way we can conceive bodies to operate in" (II.viii.11), where 'impulse' means 'transfer of motion via contact.' That is (as Locke elaborates in (12)), some quality of the external body causes some motion in our eyes, toungue, ear, skin, or nose -- a motion which is transferred in turn to our nerves, and again in turn to our brain. This motion in our brain then produces the idea that we are consciously aware of.
Primary and secondary qualities. Locke subdivides qualities into two kinds (well, really three, but we won't discuss the third).
Primary qualities are those qualities that
"are [1] utterly inseparable from the body, in whatever state it be, ... and [2] such as sense finds in every particle of matter which has bulk enough to be perceived, and [3] the mind finds inseparable from every particle of matter, though less than to make itself singly perceived by the senses" (II.viii.9).In other words, a primary quality is one that
Secondary qualities are those "qualities which in truth are nothing in the objects themselves but power to produce various sensations in us by their primary qualities" (II.viii.10). For example, "colors, sounds, tastes, etc." Secondary qualities must lack some or all of 1.-3. in the previous paragraph.
- How do secondary qualities produce their corresponding ideas? As we've said above, there is some particular motion in our brains caused by looking at the sky on a sunny day that somehow creates the idea of blue in our minds. How and why does this particular motion of brain-particles correspond to the sensory awareness of blue? (This is one modern-day formulation of the mind-body problem.) Locke's answer: God chose to "annex" certain ideas to certain motions (II.viii.13) -- just as he chose to annex the sensation of pain to the motion of a knife cutting us, even though there is no resemblance between that sensation and that motion.
- Secondary qualities "depend on" the primary qualities. That is, whether a given piece of food is sweet or sour is determined by the shape, size, and motion of the microscopic parts of that food. You can't have two macroscopic bodies that have the same primary qualities at the microscopic level, but different secondary qualities. (No difference in secondary qualities without a difference in primary qualities.)
- As evidence for his claim that heat is nothing in a body but a power to produce a certain sensation in us resulting from primary qualities, Locke provides the following example. Suppose you put your right hand close to an open flame, and you put an ice cube in your left hand, and leave both hands there for a while. Then, you put both hands in a pot of luke warm water: your right hand (the one near the fire) will feel cold, and your left hand will feel hot. This is puzzling: how can a single thing be both hot and cold, i.e., have some property and its contrary? Locke answers by citing the standard mechanical philosopher's conception of heat: the heat of a body corresponds to the motion in a body -- the faster the particles of a (macroscopic) body are moving, the hotter the body is. So with my hands in the lukewarm water, what is going on is that the water particles' average speed is between the speed of the particles in my left hand, and those in my right hand.
- As another piece of evidence that color is nothing in the body itself, Locke appeals (later, in II.xxiii.11) to observations made with a microscope. Things' colors disappear when studied under the microscope (but things under the microscope still have all the primary qualities).
Comparing our ideas of primary and secondary qualities. As just mentioned, ideas of secondary qualities do not at all resemble their causes: nothing in the sky resembles my idea of blue, nothing in the fire resembles my idea of heat. However, Locke says, my ideas of a body's primary qualities do resemble their causes -- i.e., they do resemble the shape, motion, etc. of that body. Locke writes:
"the ideas of primary qualities of bodies are resemblances of them, and their patterns really do exist in the bodies themselves, but the idea produced in us by the secondary qualities have no resemblance of them at all." (II.viii.15)And later: "a circle or a square are the same, whether in idea or existence" (II.viii.18).
Particular kinds of substance. So much for the meaning of the word 'substance.' But there are also particular kinds of substance: e.g., gold, water, horse, human, and many others. We say that gold is yellow, shiny, malleable, and so on -- but what is our idea of gold itself? A: the combination of our ideas of yellow, of shiny, of malleable, and all the rest, PLUS the supposition of some unknown thing that causes all these ideas to appear together regularly. Locke calls this unknown thing an "internal constitution or hidden essence." And we are ignorant of it (IV.iii)
- Question: do we today know the (Lockean) substances of things? Locke says (IV.III.12-13) that we have almost no knowledge of the primary qualities of things, or of how the primary qualities produce secondary qualities. But that was 1686 -- have matters changed? For example, we know the genomes of several animals, and DNA plays a large part in determining many animal characteristics; also, we know the atomic structure of gold, which explains why it is shiny and malleable.