The Limits of Experience
Hume next questions whether we possess any evidence that this principle is true, or that it has to be true. Since we can readily imagine that the world might change in many respects in the future, it is not possible to demonstrate that nature must be uniform. Our experience up to the present moment does not constitute evidence as to what the future course of nature will be, or must be. Just because the sun has risen every day up to now does not prove that it has to rise tomorrow. We can judge the future only if we know that nature is uniform. However, our information up to this point is only that, so far, nature has always been uniform. Experience can provide us with no clue about what has to be the case in the future. Hence, we can neither demonstrate nor prove from experience that the all-important principle of the uniformity of nature is true, even though much of our reasoning about the world depends upon it.
The acceptance of this principle, Hume contends, is a fundamental characteristic of human nature. We have a habit or custom that operates upon us for unknown and unknowable reasons. After we have experienced the same sequence of conjoined events several times, then, when we perceive one of the conjuncts, habit or custom leads us to think of the other, and to think of it in a lively and forceful way. Although we are able to think of any idea we wish, we are led psychologically to think only of a particular conjoined idea and to conceive of it with some of the force and vivacity of its conjoined impression. Such force and vivacity constitute our belief in the actual occurrence of the conjoined item. In terms of this explanation, the principle of the uniformity of nature is more a principle about how we think and feel than it is one about the order of events in the world.