We all act as if we believe the world behaves in a consistent and regular manner; that past patterns of behavior will persist into the future, and into the unobserved present. This persistence of regularities is sometimes called the Principle of the Uniformity of Nature, which is discussed later in this document.
Hume wrote that we could not conclusively prove the principle of uniformity in nature, because justification comes in only two varieties, and both of these are inadequate. These two types of reasoning are commonly called deductive (or a priori) and inferential/inductive (or a posteriori). The uniformity principle cannot be deduced because past regularity in nature is no guarantee of future regularity, no matter how probable we may think it is. There are no general principles inherent in past events that compel belief in the orderly progression future events. It is conceivable that nature might stop being regular at any time, as it has on rare instances in the past (consider the occasional, uncommon meteor strike, supernova, or earthquake). We can’t logically maintain that nature will continue to be uniform because it always has been up to now, because this way of reasoning uses induction to prove that induction is valid. This is circular reasoning (discussed in the Infinite Regress Problem section earlier in this document). Thus no form of logical justification will rationally warrant our inductive inferences. Yet we still believe in them.
Hume’s solution to this problem was to say that natural instinct, rather than reason, explains our ability to make inductive inferences. It is our natural instinct that allows us to connect this intuitive series of propositions together:
- In our past, the sun has risen every day
- Based on what we know about how sunrises work, there is no evidence to suggest that this will not continue to be the case
- Therefore the sun will rise tomorrow
Our expectations about such things depend on the relation of cause and effect. It is our common sense about this relation that tells us that depending on tomorrow's sunrise is a reasonable expectation. However, if all matters of fact are based on similar types of causal relations, and if all of these causal relationships depend upon induction, then we must somehow demonstrate that induction is valid.
Hume uses the fact that induction assumes a valid connection between a proposition like "today the sunrise followed a long period of darkness" and the proposition "tonight's darkness will be followed by a similar sunrise." We connects these two propositions not by reason, but by induction.
Probably the first modern philosopher to exhaustively study the problem of induction, Hume was followed by many who tried to address the problems he posed. But they still continue to puzzle us. He argued that it is just as possible to conceive of a contrary proposition to the sun rising:
- "That the sun will not rise tomorrow is no less intelligible a proposition, and implies no more contradiction, than the affirmation, that it will rise. We should in vain, therefore, attempt to demonstrate its falsehood. Were it demonstratively false, it would imply contradiction, and could be distinctly conceived by the mind."
In other words, just as we can’t prove the sun will rise, we can’t prove it won’t rise. We can conceive of either course, and neither would contradict any firmly accepted premises. As Hume said elsewhere, existence (or non-existence) cannot be proved through a priori reasoning unless one or the other would cause a contradiction. Of course, everything we know about how orbiting objects work tells us that it would be quite an unlikely feat to stop this well understood phenomenon from happening. But how do we really know that these same physical laws will persist, that the future will resemble the past? This reasoning has to be either a priori or a posteriori. Hume contended that it couldn’t be a priori (deductive). If we were to see the sun rising for the first time we would never discover from that event alone what produced it, just as a child can't use reason to stop himself from touching a flame for the first time. Only after having experienced the pain does the child learn the relation. Knowledge of such causal relations must come only through experience of the relations between objects – therefore our reasoning must be a posteriori (inferential) and require the collection of evidence, not the exercise of pure logic.
Our expectations of the future matters of fact lies in the relation of cause and effect, say both Hume and common sense. "By means of that relation alone, we can go beyond the evidence of our memory and senses". The only way we could obtain knowledge of causality would be to infer it from our past observations of regularities. Our prediction of future events based on the past observations is not a rational activity, but just a matter of habit and an intuitive sense of probability – the odds of the sun not rising are infinitesimal. When we project findings about these relations into the future, we must use an intermediate premise, the uniformity of nature, which is risky, because it can change at any time and be proven false. The chicken thinks that the human will always bring it grain until the day he comes with a hatchet. According to Hume:
- "It is impossible, therefore, that any arguments from experience can prove this resemblance of the past to the future; since all these arguments are founded on the supposition of that resemblance. All inferences from experience, therefore, are effects of custom, not of reasoning."
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