So, the message of the eighteenth–century ‘enlightened skeptics’ is clear: ‘nothing tends more to disturb our understanding, and precipitate us into any opinions, however unreasonable, than their connection with passion’ (THN,321). Or, as the poet finely put it: ‘Are passions, then, the pagans of the soul? Reason alone baptized?’

The study of man, according to this view, yields the conclusion that, although human nature is essentially uniform ‘in all nations and ages’ and stable ‘in its principles and operations’ (IstE,83), man's beliefs and opinions tend to be rather more volatile. As Hume observed: ‘Man is a very variable being, and susceptible of many different opinions, principles, and rules of conduct. What may be true, while he adheres to one way of thinking, will be found false, when he has embraced an opposite set of manners and opinions’ (E,255–6).

The whole argument (chapters 6 and 7) is aptly summarized by Mai thus' assertion that ‘The voluntary actions of men may originate in their opinions: but these opinions will be very differently modified in creatures compounded of a rational faculty and corporeal propensities, from what they would be in beings wholly intellectual’. The individual economic agent is best seen as a ‘compound being’: ‘A truth may be brought home to his conviction as a rational being, though he may determine to act contrary to it, as a compound being’.