...Hume destroys Empiricism entirely. The truth is, he says, we have no way of finding out what substance is. In fact, all of our most cherished beliefs are based on the past. For example, why do we believe the sun will come up tomorrow? Because that's what it always does. However, the sun can 'rise' every day for a billion years, and this provides no evidence that it will tomorrow - unless the universe is assumed to be internally consistent. That is, the future will be like the past. However, this point must be taken on faith; no evidence could ever brought to support this position without using circular reasoning. The end result is that the whole of reality has no rational foundation whatsoever.
The dialogue form was a favorite of Hume, and that might prove helpful in presenting his point. In my chosen example, he has run into Oscar Wilde in a temporally dubious dinner party.
WILDE: Good evening, sir. It has been reported to me that you disbelieve in cause, for which unwilling fathers should rejoice to hear.
HUME: How wonderful to meet you. Yes, it's true. I illustrate it thus: imagine a game of billiard. When the white ball strikes the eight ball, what does one see? Motion, a sound, then further motion, but never cause. We see constant conjunction - one ball 'strikes' another, apparently changing its direction - but we can never observe cause. We infer it.
WILDE: I never play billiard. It seems to be a veritable obsession among charming people, and there is nothing quite so loathsome as charming people.
HUME: It is merely an example. The point is that constant conjunction does not imply fixed regularity. The fact that two objects may exhibit movement when in close proximity with one another at a certain point of time does not guarantee that they will continue to do so.
WILDE: Ah yes. One need only view my romantic entanglements for illustration.
HUME: Quite. Mr. Wilde, that the universe is orderly can be but a convenient mode of thought; indeed, it is thoroughly undemonstrable. One can only make the circular argument: why will this chair continue to exist? Because it always has previously. And so on, with all experiences. I am afraid that all we believe is sophistry and illusion.
WILDE: (wistful) And isn't it lucky for me that it is so. For without sophistry I should never have what I desire, and without illusion they should never enjoy it.
HUME: Ahem. Yes. Pardon me, I do hearken the call of cucumber sandwiches.
There is a real-life example in favor of Hume. It used to be that one of the standard examples of an absolutely true proposition, appearing in philosophy books, was "All swans are white." It doesn't appear anymore because black swans were discovered in Australia. No matter how many instances we have of a particular event, it will never be illogical to think that something else might happen.
Now as a practical matter, rational science continues on and we do not consign all to the flames, as Hume himself suggested. We still have to make our way in this universe, and empiricism remains our best tool to obtain knowledge. However, the recognition that we cannot, in the last analysis, rationally justify our tool should teach us to be careful about our declarations. We do the best we can with what we have, but we might be wrong. In fact the odds are, we are.