In fact, although Hume's criticism applies to much of the social contract tradition, it does not apply to Hobbes. For Hobbes, it is rational to keep our word because we all benefit greatly from peace, but it is so only if we can be assured that others will do likewise. For Hobbes, this guarantee can only be provided by a Sovereign. It is the prospect of agreement, and the benefits that will bring, together with the absence of the risk of the other defecting from the agreement, that makes it rational to keep one's word and that marks the first step to overcoming the problem of trust.

In essence, this is Hume's solution, too. However, Hume puts far less emphasis on the role of the Sovereign. The reason for this can be traced back to Hume's more optimistic view of human nature. Given that we all have experience of the benefits of co-operation and agreement (in interactions with those we care about), Hume conceives of the problem of trust not as the problem of having to convince mutually antagonistic egoists to co-operate. Rather, it is the problem of reassuring persons who know of the benefits of co-operation that, if they co-operate, they will not be vulnerable to those who would take advantage of them. The solution, then, lies in each person seeing the advantages made possible by such 'artifices' as rules of property and justice. These conventions – these restraints on the unrestricted pursuit of self-interest – find approval, as Hume puts it, 'in the judgement and understanding' because of the great advantages that they make possible.

Does this solve the problem of trust? Should our two farmers, aware as they are of the great advantages of co-operation, trust one another, each confident that the other sees the benefits of the artificial virtue of justice? We might be inclined to think that it does, for surely each farmer must realise how much better off he will be if they can co-operate and jointly harvest their corn. But, there is a problem, and Hume, ever-ready as he was to confront honestly the consequences of his own thought, expressed it through the person of 'The Sensible Knave'. The Sensible Knave realizes that mutual co-operation brings great benefits, but he also realizes that on occasion he can prosper by deceit without threatening the scheme of mutual co-operation (in the modern jargon, he can 'free-ride'). Hume's answer, as we have seen, is that in general 'honesty is the best policy'. The Sensible Knave agrees that this is so 'in general', but also sees that it may not always be the best policy for him. Hume writes:

That honesty is the best policy, may be a good general rule, but is liable to many exceptions; and he, it may perhaps be thought, conducts himself with most wisdom, who observes the general rule, and takes advantage of all the exceptions.

Hume has no 'official' answer to this free-rider problem. His solution to the problem of trust turns out to be no solution at all, at least when confronted by a hard hearted knave. However, Hume thinks the knave mistaken nevertheless; not mistaken in his reasoning, but in sacrificing his character just for some material gain ('for worthless toys and gewgaws'). It is perhaps in this line of thought – in the argument that we must live with ourselves as well as with others – that a solution to the problem of trust lies.

Further Reading

David Hume, A Treatise of Human Nature. Edited by L. A. Selby-Bigge and P. H. Nidditch. Oxford: Clarendon Press, 1975 [1737].
Essays, Moral, Political and Literary. Edited by E. F. Miller. Indianapolis. IN.: Liberty Classics, 1985.
Enquiries Concerning Human Understanding and Concerning the Principles of Morals. Edited by L. A. Selby-Bigge and P. H. Nidditch. Oxford: Clarendon Press, 1978.
Martin Hollis, Trust Within Reason. Cambridge: Cambridge University Press, 1998.