Notes on Hume's Treatise
by G. J. Mattey
Book I, Part II OF THE IDEAS OF SPACE AND TIME
§. V. The same subject continu'd.
There can be no idea of a vacuum, "space, where there is nothing visible or tangible," given the thesis that the idea of space is that of the distribution of visible or tangible points.
First objection. The existence of a long controversy over the vacuum is evidence that it is a meaningful concept.
Second objection. The possibility of the idea of a vacuum is shown by the following argument. There is closure over possibility (whatever is the consequence of something possible is possible). It is possible that there be no motion of the world, and it is possible that God annihilate any part of the motionless world, everything remaining motionless. This is possible because by the separability principle, no part of matter is dependent on any other part. [Cf. p. 244, "every perception is a substance."] If this occurred, the objects would not touch each other, maintaining the relation of their parts, without motion, which is ruled out by hypothesis. So there would be a vacuum between the objects.
Third objection. The necessity of a vacuum is shown by observable motion, which would be impossible without a vacuum. This is a question of natural philosophy and will not be pursued.
To answer these questions, "we must take the matter pretty deep." Begin with the claim that darkness is nothing positive, but only "the negation of light, or more properly speaking, of colour'd and visible objects." So the idea of utter darkness is not the same as that of a vacuum (extension without matter).
There is a corresponding negation with respect to tangible ideas. If one's motion is utterly unhindered, one does not thereby get an idea of empty space through which one moves (though one may get an idea of time this way).
Therefore, neither darkness nor unhindered motion by themselves give the idea of a vacuum. Perhaps they give such an idea by a mixture: darkness/light, unhindered/hindered motion.
Reason rather than the senses discovers distance, since all objects appear as if painted on a plane surface. Thus a picture of a vacuum would be one in which luminous areas are separated by utter darkness, and do not light anything but themselves. A parallel supposition holds for touch: hindrance is followed by unhindered motion, etc.
With respect to vision, we can tell that the luminous objects are separated, that they have a certain distance, which can increase and decrease. "But as the distance is not in this case any thing colour'd or visible, it may be thought that there is here a vacuum or pure extension, not only intelligible to the mind, but obvious to the very senses."
However, though this conclusion is natural and most familiar, it is mistaken: this distance cannot produce an idea of extension because the distance is "nothing but darkness, or the negation of light; without parts, without composition, invariable and indivisible."
It must be recalled that distance is judged by reason, not sensed. There is a relation between the bodies and the visual apparatus, resulting in perceptions which give us the ability to judge distance. But these perceptions are "simple and indivisible" and do not give us an idea of extension.
The same holds in the case of what is tangible. The person whose motion is entirely unhindered feels nothing, while the one who feels nothing, then something, then nothing, etc. when moving feels only those objects. Since the sensation of motion when one feels nothing opposing it is the same in both cases, the presence of the objects in the second case reveals nothing further.
But the darkness and unhindered motion are the causes why we imagine that there is a vacuum. "For there is a close relation betwixt that motion and darkness, and a real extension, or composition of visible and tangible objects."
The similarity between objects separated by darkness and those separated by light (and mutatis mutandis for the motion case) is that the relation of the objects to the sensible apparatus is the same.
As a consequence, the distances in the two cases are convertible, "without any change on the distant objects."
The relation of the objects to other things is also unaffected by the interposition of darkness (or hindrance). The qualities, e.g. brightness, diminish with distance whether there are bodies interposed or not.
"Here then are three relations betwixt that distance, which conveys the idea of extension, and that other, which is not fill'd with any colour'd or solid object. The distant objects affect the senses in the same manner, whether separated by the one distance or the other; the second species of distance is found capable of receiving the first; and they both equally diminish the force of every quality."
Thus the two are often confused, since "we may establish it as a general maxim in this science of human nature, that wherever there is a close relation betwixt two ideas, the mind is very apt to mistake them, and in all its discourses and reasonings to use the one for the other." An explanation of this will be given, but its plausibility does not affect the truth of the phenomenon.
Here is the discussion of how the brain might be disposed to conform to the principles of association, resemblance, contiguity, and causation. An explanation of the mind's tendency to run into related ideas is that the course of the animal spirits targeted toward a specific idea naturally strays into "contiguous traces," misses the mark, and settles on a nearby related idea. We are not sensible of this, and so we use the related idea in lieu of the targeted idea. "This is the cause of many mistakes and sophisms in philosophy; as will naturally be imagin'd, and as it wou'd be easy to shew, if there was occasion."
Resemblance is the primary source of error: the actions of the mind may resemble each other so much as to be indistinguishable. Causation and contiguity may have the same influence, particularly in philosophy the confusion of words for ideas. We do this in the case of distance: "we substitute the idea of a distance, which is not consider either as visible or tangible, in the room of extension, which is nothing but a composition of visible or tangible points dispos'd in a certain order." The convertibility [see above] is "a kind of cause" and the similarity of their manner of affecting the senses and of their own behavior in the two cases is resemblance.
All objections , whether from metaphysics or mechanics , may now be answered. The frequency of disputes does not prove the reality of the idea, but testifies to the confusion just explained.
The annihilation of something between objects at rest preserves the relation of those objects to one another, and their distance, without affording an idea of a vacuum.
The motion of a body has the same effect as its creation, so the same argument applies. The fact that one body moves such that there is a visible idea where there was none before does not prove that there was an empty space: the distance relation remains the same.
These responses might be said to concern only the matter in which bodies affect the senses, "without endeavouring to account for their real nature and operations." No explanation of the causes of the phenomena (bodies separated by nothing visible or tangible) is given.
"I answer this objection, by pleading guilty, and by confessing that my intention never was to penetrate into the nature of bodies, or explain the secret causes of their operations." Not only is it beyond the scope of the present work, it cannot be done. Knowing the manner in which things affect me satisfies all practical, as well as philosophical, needs.
The conclusion consists of a paradox, easily explained by what has been said. It is a dilemma. If the invisible (intangible) distance is called a vacuum, then "extension and matter are the same, and yet there is a vacuum." If not, then motion may take place in a plenum without any of the usual expedients to explain it (e.g., "antiperistasis": things in front loop behind). The solution seems to be similar to that used by Kant in his discussion of the antinomies: to say that both sides depend on the hypostatization of distance.
The issues surrounding extension are applicable mutatis mutandis to time. Time is nothing but the manner in which some real objects exist, so can there be time without changeable existence? But we have no idea of such a thing, for there is no impression from which it is derived.
Still, we can find out the appearances which give us the mistaken notion that there is such an idea. The difference in the perceptions of an unchanging object greatly resembles what would be the case if the object were really different; a conversion is possible between them. The fictitious duration of the unchanging object stands in all the same relations (increase and diminution) to physical things as holds for the succession in the perceptions.