He is arguing that we cannot but think and reason as we do, not just that it is a practical policy to guide action by what seems probable. And, consequently, the idea of total scepticism, a total suspense of judgement, is a fantasy. There could be no such person as a total sceptic:

Nature, by an absolute and uncontroulable necessity has determin’d us to judge as well as to breathe and feel… Whoever has taken the pains to refute the cavils of… total scepticism, has really disputed without an antagonist.20

When Hume speaks of nature in these terms, he is taking a positive view of the convictions we find ourselves to have. But it is as much part of his complex theory of human nature that many beliefs arise in us which are not merely not known to be true, but positively absurd or harmful. In the Treatise he tries to separate the strong, irresistible and beneficial convictions which experience generates in the mind of the sceptical wise man (that fire burns, for example) from the fanciful, avoidable and often harmful notions that clutter the minds of the vulgar. Part of his theory is that powerful emotions often underlie the latter. We have seen that this diagnosis is important in his account of the origins of superstition and enthusiasm in religion.

The wise, who are enlightened by Hume’s theory of human nature, know themselves better than do the vulgar. They can form general, reflective rules, based on the theory, by which to guard against the fictions of a lively imagination. In fact, it is the very process of reflection which diminishes the initial credibility of the ideas presented to us by emotion and imagination. So it would seem that the more reflective we are in our mental life, the better. But there is a twist in this. If we were to attempt to govern our beliefs entirely by reflective, general principles, we would end up without any beliefs at all – precisely the Pyrrhonian desideratum. For, as the sceptics showed, general principles of logic and methodology can always be set against one another so as to pull us in opposite directions. Hume’s most considered view is that the beliefs of the wise man will be a vector of experience and reflection.

When Hume says that reason, in the sense of the capacity to infer effects from causes and vice versa, is an instinct, he goes on to offer an account of how it operates. Very roughly, he thinks that we initially believe the evidence of our senses and memory, which give us vivid and forceful impressions. When we have had experience of the constant conjunction of two phenomena, such as fire and heat, the ideas of these become associated in the mind by a kind of conditioning. Hume calls this ‘habit’ or ‘custom’. Then, whenever we have an impression of one of these phenomena, as when we see fire, the idea which is habitually associated with it, such as heat, immediately enters our consciousness. We see the child put her hand in the fire, and at once think that she will be burned. The belief we have in the evidence of our senses, which Hume identifies with the vividness and force of our perceptions, is transferred to the associated idea, and we believe that too. We do not merely think that she will be burned, we believe that she will. So belief itself turns out to be a naturally produced psychological state, which consists essentially in a kind of feeling: ideas which are the contents of belief feel strong, forceful and vivid.

It is because all this happens, he thinks, naturally and inevitably in everyday cases, like the example of the fire, that the Pyrrhonian idea that we might always suspend our assent, and have no beliefs at all, is absurd. This general position about how far it is possible to maintain a sceptical suspense of judgement is laid out at the beginning of the Dialogues, in the discussion between Cleanthes and Philo in Part I. But, very much in line with Hume’s view, Philo claims that a sceptical suspense is possible where we are considering only abstract, reflective arguments. In those circumstances, the sceptic can always argue against the thesis being considered, so producing a weakening of conviction to balance the persuasive force of the original argument. If the topic is one on which we have no strong impulse to hold one view rather than another which results inevitably and unreflectively from past experience, then the dialectic of arguments for and against will lead to suspense of judgement:

All sceptics pretend that, if reason be considered in an abstract view, it furnishes invincible arguments against itself, and that we could never retain any conviction or assurance, on any subject, were not the sceptical reasonings so refined and subtle that they are not able to counterpoise the more solid and more natural arguments derived from the senses and experience. But it is evident, whenever our arguments lose this advantage and run wide of common life, that the most refined scepticism comes to be upon a footing with them, and is able to oppose and counterbalance them. The one has no more weight than the other. The mind must remain in suspense between them; and it is that very suspense or balance which is the triumph of scepticism.

Philo intends to show that in natural theology, our arguments do run wide of common life. But he is prepared to find himself convinced by very ‘refined’ and ‘abstract’ reasoning in, say, Newton’s writings.