Hume’s critique of the modern philosophy
Hume's critique occurs in Book 1, Part 4, Section 4 of the Treatise of Human Nature (T 1.4.4), mainly in paragraphs 3 through 10 (T 1.4.4.3—10). Here is my reconstruction of Hume's argument, along with commentary on each of the premises:
1.
There are no colors or other secondary qualities in bodies (paragraphs 3 and 4).Colors, sounds, and so on are only impressions in the mind, resembling nothing in bod- ies.This is the first main tenet of the modern philosophy. Some readers may think Hume is assuming it only as a premise for a reductio ad absurdum, but Hume rehearses the standard arguments for it and pronounces them “as satisfactory as can possibly be imagin'd” (T 1.4.4.4). For example, the distant mountain looks blue in the haze (it presents me with a blue impression), but when I get close enough to hike on it, I see that it is mostly green (it presents me with green impressions). Since the mountain itself did not change as I approached it, not all my color impressions resemble it, and the modern philosophy concludes that none do.
2. Bodies have no properties but the primary qualities—extension, motion, and solidity (and others that entail one or more of these, such as figure) (paragraph 5).
This is the second main tenet of the modern philosophy, telling us what remains once the secondary qualities are removed.
What is now to be shown is that if there are no secondary qualities in bodies, we cannot conceive of bodies as having primary qualities, either—in which case we cannot conceive of bodies at all (paragraph 6).
We need to distinguish between two things that might be meant by saying such and such is not conceivable. On the one hand, it might mean that we can form no concept of such and such—in Hume's terms, that we have no idea of it. In his philosophy, this is most often the case when we have no impressions from which the idea might be derived.
On the other hand, it might mean that some state of affairs is inconceivable in the sense that Hume takes to imply the impossibility of it—the sense that figures in his maxims that what is inconceivable is impossible and what is conceivable is possible (T 1.2.2.8 and many other places in the Treatise). It should be clear that something's being inconceivable in the first sense (inconceivability1) need not imply its being inconceivable in the second sense (inconceivability^. A blind man cannot conceive of the color red, but no one should conclude that it is impossible for there to be red things.To anticipate, Hume's conclusion is going to be that under the modern philosophy, bodies are inconceivable1.3. We can conceive of bodies only if we can conceive of them as having (some of) the properties they actually have.
Hume does not explicitly state this premise, but there is a hint of it in T 1.4.4.10, and it is required for the validity of his argument. The alternative to admitting it would be very strange—it would be to hold that there are things we can conceive of only by putting them in a false light (or by not attributing any properties to them at all).
4. We can conceive of bodies only if we can conceive of them as having one or more of the primary qualities—motion, extension, solidity.
This step, presupposed throughout Hume's argument, follows from 2 and 3. He is not forgetting other primary qualities such as shape—shapes are ways of being extended and cannot be conceived without extension.
5. We can conceive of a body as moving only if we also conceive of it as extended or solid (paragraph 7).
Hume argues for this premise as follows:
[Motion] is a quality altogether inconceivable alone, and without a reference to some other object.The idea of motion necessarily supposes that of a body moving. Now what is our idea of the moving body, without which motion is incomprehensible? It must resolve itself into the idea of extension or of solidity.
(T 1.4.4.7)
Why not of color? Because, as the next paragraph reminds us, color is excluded from bodies by the modern philosophy.
6. We can conceive of a body as extended only if we also conceive of it as either colored or solid, that is, conceive of it as colored or conceive of it as solid (paragraph 8).
In support of this premise Hume cites his own analysis of extension, given most explicitly at T 1.2.3.4—5. An extended thing is an array of indivisible points, and if these points are to be more than non-entities and amount to anything through their aggregation, they must be either colored or solid.
7. “Color is excluded from any real existence” (paragraph 8).
This is just a reaffirmation of the first tenet of the modern philosophy. Colors belong only to impressions and ideas in our minds, not to any external bodies.The same goes for heat and cold and any other secondary qualities through which we might attempt to get a purchase on the extendedness of bodies.
8. We can conceive of a body as extended only if we also conceive of it as solid (paragraph 8).As Hume puts it,“The reality, therefore, of our idea of extension depends upon the reality of that of solidity, nor can the former be just while the latter is chimerical.”
At first glance, it may seem that 8 follows from 6 and 7, perhaps with the help of 3. On closer inspection, however, there is a non sequitur, which I am not sure how to repair.We would need a premise stronger than 3—one implying that we can conceive of a body as colored only if it actually is colored. But that is too strong—wouldn't Hume allow that we can conceive of bodies as colored, even if the modern philosophy is correct in arguing that they are not colored? Alternatively, we could make it a premise that we can conceive of bodies as colored only if it is possible for them to colored, adding that the modern philosophy rules out that possibility.1 But I doubt that Hume credits the argument from independent variability with that much power—it only shows that bodies are not in fact colored, not that they could not be.
I leave the proper rationale for 8 as an unsolved problem.9. We can conceive of bodies only if we conceive of them as solid.
This follows from 4, 5, and 8. Solidity is a property without which we cannot get any mental grip on what bodies are. Many of the moderns, including Locke, Reid, and Kant, would have agreed with Hume on this point.
10. There is no way to understand solidity but this: to be solid is to resist penetration by other solid things (paragraph 9).
Hume equates solidity with impenetrability:“The idea of solidity is that of two objects, which being impell'd by the utmost force, cannot penetrate each other; but still maintain a separate and distinct existence.” The equation was common in Hume's day. For Locke, solidity may have been the categorical basis of impenetrability rather than being identical with it,2 but it was still a property that could only be understood in terms of impenetrability, which is all Hume's argument requires.
Hume does not say in the sentence I quoted that to be solid is to be impenetrable by other solid things, but he arrives at that claim in short order. How are we to conceive of the other bodies by which a given body is impenetrable? Not by way of any secondary qualities, since these are excluded from bodies. By way of motion or extension, then? But according to premises 5 and 8, that would require that we conceive of those other bodies as being solid.Thus we can understand solidity only as a relation to other solid things.
11. If 10 is true—if we can understand solidity only in terms of solidity—we do not understand solidity at all.
So what if we can understand solidity only in terms of itself, some may ask at this point—that just shows it is a primitive concept. Relatedly, there are sometimes small circles of mutually definable concepts where there is no prospect of a definition taking us outside the circle. That does not mean that no concept in the circle is understood; it just means that they are understood only if at least one is understood without need for definition.
But Hume would deny that that is our situation here. It really is true that we can understand solidity only if we have a prior grasp of body or some other bodily mark, and that we can understand body only if we have a prior grasp of solidity. If that is the case, we do not understand either of them. As Hume puts it:‘Tis impossible, therefore, that the idea of solidity can depend on either of them. For that wou'd be to run in a circle, and make one idea depend on another while at the same time the latter depends on the former.
(T 1.4.4.9)
12. ‘Our modern philosophy, therefore, leaves us no just nor satisfactory idea of solidity; nor consequently of matter,(paragraph 9).
This is the conclusion I am entitling Humean Humility. It is not the thesis that it is inconceivable2, that there be bodies, but the thesis that we can form no satisfactory conception of what they are. We should contrast this conclusion with two other conclusions that Hume sometimes seems to draw, one weaker and one stronger.
In announcing his conclusion at the start of the argument, he says that by depriving external objects of their primary qualities, “we reduce ourselves to the opinions of the most extravagant skepticism concerning [external objects]” (T 1.4.4.6). If by skepticism is meant the doctrine that we cannot know that external objects exist, this gloss is an understatement. Humean Humility is not a doctrine about what we can know, but about what we can even conceive of or entertain. It is conceptual skepticism rather than epistemic skepticism. Hume offers reasons for epistemic skepticism elsewhere (e.g., in T 1.4.2), but the upshot of 1.4.4 is conceptual skepticism.3 His next sentence gets it right: “If colours, sounds, tastes, and smells be merely perceptions, nothing we can conceive is possest of a real, continu'd, and independent existence; not even motion, extension and solidity, which are the primary qualities chiefly insisted on” (T 1.4.4.6). Since having a continued and distinct existence is the mark of body in T 1.4.2, he is saying we cannot conceive of bodies.
The final sentence of the section gives another gloss of his conclusion, this one overstating it: “When we exclude these sensible qualities there remains nothing in the universe, which has [continu'd and independent existence].”
(T 1.4.4.15)
Here he advances beyond skepticism of either variety to nihilism: there are no bodies.This sentence is probably best taken as exaggeration for dramatic effect.