In this excerpt, Clark demonstrates that empiricism reduces to skepticism. Headings have been added to the text to help with following the logic of Clark's arguments.
According to empiricism, knowledge begins with what Locke calls ideas, notions, or phantasms, what Hume calls impressions, or what most people today call sensations. By combining, transposing, augmenting, or diminishing these sensory materials, we develop all, even our most abstruse, knowledge. The most complicated of Einstein's relativity equations, though this is not the example Hume used, can be resolved into memory images that were copied from previous sensations.
Now, how far will experience take us? Do these inner sensations give any knowledge of external bodies? Can we discover the causes of these impressions? Berkeley had already showed that the sensations of red, hard, bitter, etc., give no evidence in favor of the existence of an external material world. Hume, following him, gives the example of a table. Suppose we see a table. We have a sensation of a table. If we walk away from it down a long hallway, what we see appears smaller than what we saw when we were closer. An external table is supposed always to maintain the same size. Therefore what we actually saw was not the alleged external table, for what we actually saw changed in size. What we actually saw was an image or phantasm in our own mind, and hence our sensations furnish us with no evidence for the existence of an external world. Even if we should suppose that our image had some external cause, we could not know that the image resembles the cause, for we have seen nothing but images. In fact, if the word image connotes a similarity to something external, we have no reason to believe that our sensations are images.
Hume, however, goes further than Berkeley in reducing knowledge to experience. Locke had accounted for the idea of matter by abstraction, and Berkeley had shown that experience provides no instance of an abstract idea. Ideas of blue, red, and green we have in abundance; but the abstract idea of a color that is neither blue, red, nor green—an idea of color that is no color at all—simply does not exist. Similarly, "matter" does not exist; it is merely the sound of our voice, nothing more than an empty word. But if the abstract idea of material substance is nothing, it follows with the same necessity that experience can give us no idea of spiritual substance. The one is as abstract as the other. That is to say, a mind or soul does not exist. Experience gives us ideas only. There are reds, greens, bitters, sweets, roughs, smooths and their compounds—rivers, trees, and tables; but there is neither matter nor spirit, for perception can never furnish evidence for anything imperceptible. We ourselves are nothing but a collection of sensory perceptions.
Next, if it is obvious that perception can furnish no evidence of any imperceptible entity, it is only a little less obvious that perception can furnish no evidence for what is unperceived.
If I offer a perceived letter as evidence that my unperceived friend is in France, I am assuming that there is a necessary connection between the letter and my absent friend. Were there no necessary connection, had my friend not written the letter, were he not the cause and it the effect, then I could not know that he was in France. All questions of history, therefore, in fact all alleged knowledge of facts beyond present sensation and the records of our own memory, depend on the principle of causality.
An examination of experience, however, shows that a knowledge of cause and effect is not to be had. We may have the sensations of red and a moment later a bitter taste; or the sensation of a loud noise may be followed by a sweet smell. Experience provides the succession of ideas; but we never see, smell, taste, or hear a necessary connection. There is no reason to believe that red causes a bitter taste or that a noise causes a smell. Quite the contrary, no one can image how or why a color might cause taste. This remains true for compound ideas as well as for simple ideas. The combination of white, a cubical shape, and a crystalline structure that we call sugar may precede a sweet taste. But can anyone show a necessary connection between the first set of ideas, singly or together, and the sweet taste or the feeling of a full stomach after eating quite a bit of it?
Experience accustoms us to expect certain sequences. They become so familiar we take them for granted. We call them causes and effects. But in it all we have no understanding of the sequence and no experience of any necessary connection. A knowledge of history is therefore impossible.
Now, finally, if it is impossible to know the imperceptible by perception, and if it is impossible to know the unperceived by perception, is it even possible to know what we now see? Granted that there is no evidence in experience of an unexperienced table whose size does not change, can we have even the image of a table, composed as it is of sensations of color, shape, and hardness?
Here is the difficulty. At any finite time, no matter how short, we experience a multitude of sensations. We see dozens of colors we may hear two or three sounds, we could smell several odors, and even if we have no tastes at the moment, we always have a number of tactual sensations. From this manifold of sensations we select a few and combine them to make the image of a table. But why is it that we combine the color brown, a somewhat rectangular shape, and the sensation of hardness to make a table, instead of selecting from our many sensations that color pale green, the sound of a C sharp, and the smell of freshly baked bread to combine them into the idea of a jobbleyclick?
Locke had tried to justify the connecting of certain ideas on the ground that they were qualities inhering in the same material substance. But as material substance does not exist (even if it did we could not know it until after we had combined simple ideas into things and then done some abstracting), this explanation is not available to empiricism. Berkeley and Hume give the impression that our selections for combinations depend on the fact that the ideas selected occur at the same place and time. Time, however, is unimportant, for at any time we are experiencing many ideas that we do not combine into a table. Then, must an empicirist say that the particular combination depends on the space in which the simple ideas are perceived?
Whether this answer is satisfactory or not depends on the empirical account of how we can recognize space.
Do we see space? Do we hear space? Do we smell space? Not only is this impossible, but even when we see a single object in space, we cannot see the distance between it and us. We judge distances by comparing known objects. Since we have previously seen and touched a particular table, and thus know its size at close range, we can judge how far away we are when it appears half its previous size. Or, we can judge that a house down the road is a mile away because on other occasions we have walked the distance. Space and distance therefore are matters of judgment and comparison, not of simple sensation.
But if space is learned by comparing houses and tables, we must first be able to perceive the table before we can compare it with a house and learn of space. That is to say, space is an idea of comparison. But if the idea of space cannot be had until after we have compared tables and houses, we cannot produce tables and houses by selecting simple ideas through the use of space.
Empiricism therefore has blundered fatally. It has surreptitiously inserted at the beginning of the learning process an idea of space which does not exist until after the process has been well nigh completed. Once again, then, the attempt to found knowledge on "reason" as distinct from revelation has failed. If this were the end of the story, the Christian could offer the world a choice between faith in revelation or an abysmal skepticism.