It is appropriate, therefore, to bear in mind some aspects of Hume’s attitude towards scepticism.

Scepticism was a classical Greek philosophical method, taking various forms over a long period, from around 300 BC to about AD 220. At the end of this period, the aims and methods of the sceptics were summarized by Sextus Empiricus in Outlines of Pyrrhonism. This work appeared in translation towards the end of the sixteenth century, and from that time scepticism played a major role in the development of philosophy. Pyrrhonism, which takes its name from Pyrrho of Elis (c. 360—275 BC), was the most extreme form of scepticism; Hume sometimes calls it ‘total scepticism’. Another source of knowledge of classical scepticism was Cicero’s Academica, written in or after 45 BC. The Academy was the school of philosophy founded by Plato. Some considerable time after his death, scepticism was introduced and developed by successive heads of the school. Originally the scepticism of the Academy was total, but in time a more dogmatic position emerged, partly through the influence of the rival school of Stoicism. (Sceptics and Stoics are compared in Part I of the Dialogues.) One of those responsible for this was Philo, with whom Cicero studied scepticism when Philo was in Rome. Later, more full-blooded Pyrrhonism was revived, outside the Academy, and it is this later revival that is represented in the writings of Sextus Empiricus.

Just how Pyrrhonian and Academic scepticism differed is a matter of scholarship. But Cicero certainly suggests a difference, and Hume, from his study of Cicero, learned to use the terms ‘Pyrrhonian’ and ‘Academic’ to mark a distinction. Sceptical reflections often begin with the observation that the search for truth is frustrated by the disagreements found amongst men. Experts so often tell us different things; and even in common life we find that people’s perceptions differ radically. When we are tossed about in the sea of conflicting opinions, the sceptic offers an escape to tranquillity of mind, by leading us to suspend our assent from any judgement. He does this, according to Sextus, by arguing both for and against any thesis in such a way that the issue seems undecidable. Faced with a balance between pro and con, the wise man does not commit himself either way.

Now Cicero implies that the Academic sceptics went so far towards committing themselves as to deny that knowledge can be achieved. (Sextus criticizes this as dogmatic.) And they were attacked by the Stoics, by the argument that if knowledge is impossible, then so is action. If we cannot know anything, we cannot know what to do. In response, the Academics formulated a notion of ‘probability’. (The term in this context translates Cicero’s ‘probabile’, which in turn translates the original Greek expression.) The idea was that, although we have no sure criterion by which to sort truth from error, yet some things strike us as plausible or convincing, and, without committing ourselves to thinking that they are true, we can direct our actions by them. Probability is all we need, in practical terms.

Hume endorses this general position, and uses it to criticize the total scepticism of the Pyrrhonians. But his basis for the position is his own, and not anything he found in Cicero. It is to be found in his theory, already mentioned, that reason is a kind of natural instinct. On the one hand, this is a sceptical position. Hume denies that we can provide any antecedent justification for reasoning as we do from causes and effects. And, since that is so, we have no grounds for assuming that the conclusions we reach must be true. On the other hand, we do find ourselves, so to speak, persuaded by such inferences. Since it is our nature to reason as we do, there is no alternative. Hume’s view here is much stronger than the classical sceptics.