Hume said that all our inferences from past observations to beliefs about the future "proceed upon the supposition that the future will be conformable to the past." He then went on to show that we have no reason to believe this supposition. Like Edwards, I believe that the truth of this proposition depends entirely on what is meant by "reason". However, unlike Edwards, I believe that once we uncover what Hume means by "reason", we will find that his claim is in fact a substantive one and that he is in fact saying something meaningful both about the way the human mind and the world work. Therefore, in this paper I shall try to explain why Hume thinks we need this supposition and what he means by reason, in attempt to understand why and how Hume is right in his claim.

Before diving into the topic at hand, let me say a few words about Hume's general structuring of human reasoning. Hume divides all human knowledge into two exhaustive categories: "relations of ideas" and "matters of fact". The former consists of propositions whose justification lies outside of experience and whose truth is therefore "demonstratively certain." In other words, propositions whose opposite implies a contradiction, or is inconceivable fall into this category. Since their justification lies outside experience, no experience can either prove them false or provide an exception. All other propositions, those which are justified by empirical evidence, are termed matters of fact. The opposite of a matter of fact is always conceivable and hence never implies a contradiction.

With this division in mind, we can tackle Hume's first important claim regarding our problem: that is, that all inferences or expectations about the future are necessarily based on matters of fact, and not relations between ideas. This is because, to put it bluntly, nothing that conflicts with our "common sense" expectations is inconceivable. In other words, we can at least imagine, when we have an expectation about a future event that something else could occur. Thus when we are about to let go of an object we are holding in our hand, although we believe that it will fall to the ground, we can conceive of it doing any number of other things: staying at rest, moving upwards, disappearing, etc. Any reasoning of this type Hume calls moral reasoning, which is opposed to demonstrative reasoning, or that which concerns relations between ideas.

This means that any inferences we make about the future, or any beliefs at all we have about the future, have to be based on experience, or empirical evidence. So the next step is to try to see how Hume thinks we use experience to make such inferences. He says that the way we do this is by noticing that certain events are always coupled with others, and that although all events are distinct, when we see similar events following each other time after time, we imagine that the two are connected and we form the idea of cause and effect. He says that all our arguments concerning existence, or matters of fact, are based on the relation of cause and effect. And, as we already pointed out, this relation is based on experience, not on reasoning alone. However, this is where Hume discovers the problem at hand: the only way we can use the notion of cause and effect to predict future events is to assume that the future will resemble the past. But any attempt to prove this supposition, Hume argues, is necessarily circular. Thus, all our inferences from past observations to beliefs about the future are based on an unfounded supposition.

It is rather easy to see why we need such a supposition, and we can see this by appealing to simple common sense. Its just a matter of understanding what Hume means by the future conforming to the past. He does not mean that the future will be exactly like the past, for we do not in fact believe this. However, if we imagine a future that in no way resembles the present, then it is obvious that our predictions will not hold. For example, imagine a future where bodies do not fall down when you let go of them, but instead stay still. In such a scenario, a prediction that an object would fall would be wrong. Or, let's take this a step further and imagine a future where nothing existed at all (which is not like the past, where many things existed,) then no prediction about any event at all would hold. Clearly, any prediction we make, or any expectation we have, rests not only on the supposition that the universe will continue to exist, but also on the supposition that the general laws we believe about the universe will continue to hold. Again, if we were told that in the future, no event would have a cause, for example, and that things would just happen at random, we would have no trouble understanding why we could not make any inferences about the future based on past observations.

The more difficult thing to see is why Hume believed that this supposition, which we all hold, is unfounded. To explain this, let us return to the distinction between demonstrative reasoning and moral reasoning. The supposition that the future will resemble the past cannot be shown demonstratively, since it does not imply a contradiction to state that the future will not conform to the past. Therefore, the only option left is moral reasoning, or reasoning based on experience. However, all our experience tells us is about things we have experienced in the past, and any attempt to use this experience to tell us something about the future will necessarily lead us in a circle.

We can see why this is so by observing the following line of thought: suppose we reason that we know that the future will conform to the past, because, in the past, the future has always resembled the past. This is true. In the infinite number of moments we have experienced in our lives, the following moment has resembled the preceding moment in all the ways we consider important: things have continued to exist, like events have been followed by like events, which we have termed their "causes", and, most importantly, we have been able to make successful predictions based on this uniformity. However, this doesn't prove anything, since, again, all this was in the past and tells us nothing about the future. As Russell points out, just because "past futures" have resembled "past pasts" how do we know that "future futures" will resemble "future pasts". Of course, we could try and go a step further and say that past past pasts have resembled past past futures, but this would plainly lead nowhere.

As frustrating as this may seem, and as much as we might like to say that Hume has the wrong conception of "reason", I think that his argument is irrefutable, and this is where we must acknowledge the substance of Hume's argument. This is because no matter what concept of reason we might adopt to convince us that we have a reason to believe that the future will conform to the past, that fact remains that if the future did suddenly drastically change in one of the ways mentioned above, then any justification we had for our belief would be undermined, for reason itself might cease to exist. That sounds a bit weird, but can we not conceive of a world without reason?

In conclusion, if we define "reason" as "explanation", we might be able to show why we hold on to this supposition: we simply cannot do without it. If we do not assume that the future will conform to the past, then we would have to reason to do or think anything at all. We wouldn't even be able to assume that a decision to perform a simple act like moving would cause our bodies to move, as it has always done in the past. However, this type of "reason" is a weak one, since it does not in any way exclude the possibility that we are wrong in our belief, and herein lies the substance of Hume's argument: no matter how we justify to ourselves the supposition that the future will conform to the past, in the conceivable event that this stopped being the case, we would have no reason to believe anything at all, let alone make inferences about the future (including, of course, any inference about the future conforming to the past.)