In the next lecture, I will give some reasons why Hume was not satisfied with his condition after his inquiries. In the meantime, I will note a major difference between the results of Descartes' solitude and those of Hume's. Descartes decided soon enough that he was not alone, that along with his own existence, the existence of God is certain. And he could use the existence and nature of God as a basis for claiming knowledge about the other things making up the material world. Further, he placed his faith in rational intuition, a kind of mental vision which recognizes the truth in clear and distinct perception. Hume diagnosed mental "vision" as akin to hallucination.
Hume began by examining the nature of ideas, the objects of the understanding. He claimed that we can observe that to every idea (or, as he was to qualify it, every simple idea) there corresponds an original impression. The impressions are those of sense (sights, tastes, sounds, smells, etc.), bodily states like hunger and thirst, and more generally pleasures and pains. The impressions are distinguished from ideas by being always more vivid. Ideas are copies of impressions. They can either be the result of memory or imagination. Ideas of memory are more vivid than those of imagination, and they are "tied down" to a definite sequence.
It is observable, Hume held, that ideas are simple or complex. My idea of an apple is a complex idea, whose simple ideas are those of red, a crisp texture, sweet taste, etc.
Having given this broad view of the nature of ideas, Hume turned to philosophical questions. How do we use ideas to characterize things? On the traditional view, we represent things as substances. Aristotle said that the individual human being is a substance, whose essence is to be a rational animal. Descartes had said that he is a thinking substance, and that physical objects are extended substances. So the notion is that substances are the primary beings, and that they have properties (sometimes called qualities or modes), such as thinking.
Hume abandoned this traditional view. When we examine our idea of an individual thing, all that we find is the simple ideas which go together to make a complex idea. He redirected the attention of philosophy away from substances and properties and toward relations. That is, philosophy is concerned with the relations ideas have to one another. When we compare two ideas, we find that the idea of one apple is of a deeper red than that of another. The first apple is closer to my hand than another, and so forth. The most important relation of ideas is cause and effect. This is because it takes us "beyond our senses, and informs us of existences and objects, which we do not see and feel" (p. 349). The growing of the crops in the fields this summer is something beyond our present experience, yet by reasoning according to the relation of cause and effect, we can say that the sun will make the crops grow (all else being equal). The sun is the cause and the growing crops the effect.
The question about cause and effect is, can we trust our reason in making inferences from conditions that do exist to something which does not exist? One possibility is that we have knowledge of a necessary connection between the two. If the first occurs, the second must occur. Perhaps we have a rational insight into the connection, as Descartes might have it. Hume counters by stating that we cannot use alleged necessary connections as the basis of an inference to what does not exist. The reason is that no matter how constantly the two have occurred together, we can always conceive the possibility that the first occurs without the second. I can imagine that plants fail to carry out photosynthesis, even though nothing else about them has changed, for example.
In response, it can be stated that our causal inferences are based on experience. We can infer that something will occur because it is part of a pattern which has been observed to hold, perhaps among all individuals of the same kind. Plants have never failed to convert light into usable energy when exposed to the sun (all things being equal; they have water, nutrients, a warm temperature, etc.). This kind of inference was called, in the last lecture, inductive. Many philosophers of the period held that inductive inference is the primary form of scientific reasoning.
It is here that Hume applied a unique argument against inductive inference. The assumption is that past patterns will repeat themselves. It would not matter how many cases had been observed to be the same, if there were no reason to think that future cases would follow the pattern. But what evidence do we have that present patterns will be observed in future cases?
Once again. there are two possibilities. That we have an intuition of a necessary connection is discounted again. We can always imagine that past patterns will not continue to dupicate themselve in the future. So only experience remains. It has always been my experience that past patterns repeat themselves. I observed at one time that every A is followed by a B, and I inferred that the next one would be a B. This inference was successful, in that the next A was a B. This happened many times, so I can say that every time it was the case that all A's were followed by B's, the next A was followed by a B.
Not surprisingly, Hume asked for the basis of the inference from the fact that patterns have always repeated themselves in the past to the conclusion that they will continue to do so. The same old candidates are brought out and defeated for the same reasons. There is no necessary connection, and the repeated success of this, higher-level, pattern could only be projected onto the future by the use of an even higher-level pattern, with no end.
Hume concluded that there is no justification for inductive inference, and that inductive inference is not a rational process. Instead, we believe what we do about non-existent things on the basis of mere custom and habit. We are accustomed to find patterns repeated, and we consequently form the habit of expecting that they will continue to be repeated. So our beliefs are more a matter of pure conditioning than the consequence of rational deliberation.
This led Hume to a radical new theory of the nature of belief. Descartes had held that belief is a choice, and that the ability to believe or to refuse to believe is the highest form of human freedom. For Hume, there is no choice at all, only conditioned reflex. A belief, on his view, is nothing more than a very intense idea. If I am crossing the road and see a car coming at me fast, I believe I will be hit. There is no deliberation, only a heightened intensity of the idea of my being run down.
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