I have found that the argument of Treatise 1.4.4 makes no sense on its own. This page includes material that I emailed to members of the class the day before I lectured about Treatise 1.4.4 that I thought was necessary to fill in the story. On Saturday, 29 January, I updated the page to include material on Locke, who appears to have been Hume’s target. On Monday, 31 January, I added another paragraph to the Locke section.
In Treatise 1.4.4, Hume was relying on an argument about so-called secondary qualities, such as color, taste, heat, and smell. The modern philosophy tries to draw a distinction between the qualities that objects have and the qualities that we (falsely) think they have. The former are called primary qualities, the latter secondary qualities.
So, for example, according to the modern philosophy, objects have the qualities studied (roughly) by the physical sciences: extension (dimensions in size, what you would measure with a ruler), solidity (desks have more of it than pools of water), figure (round, square, etc.), motion (obvious enough), gravity (ditto), and cohesion (they hold together).
By contrast, according to the modern philosophy, the following qualities do not exist in objects; we perceive them only because of features of our psychology and perceptual organs: colors, smells, tastes, heat, cold.
The distinction in Locke
John Locke distinguished primary and secondary qualities in An Essay Concerning Human Understanding, Book 2, chapter 8. For Locke, an object’s qualities are powers to cause ideas in our minds. Some of these qualities (solidity, extension, figure, motion, and texture) are “utterly inseparable from the Body, in what estate soever it be; such as in all the alterations and changes it suffers … it constantly keeps; and such as Sense constantly finds in every particle of Matter, which has bulk enough to be perceived …” (Essay 2.8.9). Secondary qualities (“colours, sounds, tastes, etc.”), by contrast, are “nothing in the Objects themselves, but Powers to produce various Sensations in us by their primary Qualities, i.e. by the Bulk, Figure, Texture, and Motion of their insensible parts” (Essay 2.8.10).
The distinction concerns the different qualities’ powers. Variations in the secondary qualities are explained in terms of variations in the primary qualities but not vice versa. If you crush an almond, it will look and taste different than it did before it was crushed. But the only thing that changed about the almond was its texture, a primary quality, and that explains the change in the secondary qualities of taste and color (Essay 2.8.20). Conversely, a gem’s ability to appear colored depends on whether the lights are on or not; changing this power doesn’t change anything about the gem itself (Essay 2.8.19). (Light, presumably, has primary qualities as well, such that variations in available light explain variations in the gem’s color).
Hume seems to accept one of Locke’s arguments for the distinction between primary and secondary qualities. It concerns the greater variability of perceptions of the secondary qualities.
Ideas being thus distinguished and understood, we may be able to give an account how the same water, at the same time, may produce the idea of cold by one hand and of heat by the other; whereas it is impossible that the same water, if those ideas were really in it, should at the same time be both hot and cold: For if we imagine warmth, as it is in our hands, to be nothing but a certain sort and degree of motion in the minute particles of our nerves, or animal spirits, we may understand how it is possible that the same water may, at the same time, produce the sensations of heat in one hand, and cold in the other; which yet figure never does, that never producing the idea of a square by one hand, which has produced the idea of a globe by another. But if the sensation of heat and cold be nothing but the increase or diminution of the motion of the minute parts of our bodies, caused by the corpuscles of any other body, it is easy to be understood, that if that motion be greater in one hand than in the other; if a body be applied to the two hands, which has in its minute particles a greater motion, than in those of one of the hands, and a less than in those of the other, it will increase the motion of the one hand, and lessen it in the other, and so cause the different sensations of heat and cold that depend thereon. (Locke, An Essay Concerning Human Understanding, Book 2, Ch. 8, Sect. 21)
Locke was aware that our perceptions of a thing’s size (a primary quality) vary with our distance from it. So his argument was not that our perceptions of primary qualities never vary. His argument was that we do not perceive a thing as having incompatible primary qualities at the same time, whereas we do sometimes perceive a thing’s having incompatible secondary qualities at the same time. The explanation of the latter phenomenon has to do with features of the perceiver rather than the perceived object: I perceive the temperature of the water differently in my right and left hands because my hands start out at different temperatures before touching the water. That’s a feature of me rather than a feature of the water.
Hume and Berkeley
Hume argued that we have no idea of bodies without the secondary qualities, especially color. What is your idea of a colorless body? We’ve seen the point before. See 1.1.7 paragraphs 6 and 10.
In 1.4.4, Hume was taking that point for granted. More specifically, he’s taking a particular argument made by Bishop George Berkeley for granted. Here it is.
“They who assert that figure, motion, and the rest of the primary or original qualities, do exist without the mind, in unthinking substances, do at the same time acknowledge that colours, sounds, heat, cold, and such like secondary qualities, do not, which they tell us are sensations existing in the mind alone, that depend on and are occasioned by the different size, texture, and motion of the minute particles of matter. This they take for an undoubted truth, which they can demonstrate beyond all exception. Now if it be certain, that those original qualities are inseparably united with the other sensible qualities, and not, even in thought, capable of being abstracted from them, it plainly follows that they exist only in the mind. But I desire any one to reflect and try, whether he can, by any abstraction of thought, conceive the extension and motion of a body, without all other sensible qualities. For my own part, I see evidently that it is not in my power to frame an idea of a body extended and moved, but I must withal give it some colour or other sensible quality which is acknowledged to exist only in the mind. In short, extension, figure, and motion, abstracted from all other qualities, are inconceivable. Where therefore the other sensible qualities are, there must these be also, to wit, in the mind and nowhere else.” (Berkeley, The Principles of Human Knowledge. Pt. 1, Sect. 10)
The bulk of Hume’s argument in 1.4.4 is devoted to showing that we don’t have an independent idea of solidity that could be used to answer Berkeley’s argument.
That is why he goes through the complicated assertion that the idea of motion depends on the idea of body, the idea of body depends on the idea of extension, and the idea of extension depends on the idea of solidity.
Hume, like Berkeley, thinks that we can’t have an idea of colorless extension — a length of something with no color. What would it look like?
Someone might say “we can understand extension in terms of solidity and that would be to have an idea of extension without involving color.” I’m not sure if that’s to the point or not, but Hume’s reply is that we do not have an idea of solidity capable of making this reply work: our idea of solidity is pretty much the same as our idea of extension.
At the end of the section, Hume denied that we have sensory impressions of solidity that are different in kind from our sensory impressions of the putative secondary qualities. Just as perceived colors vary from person to person, so do perceptions of solidity.
It is unclear to me whether either argument is effective against Locke’s way of drawing the distinction.