Since transference of property, and more generally rights, involves usually spoken or written interaction between human beings, Wolff also considers obligations regarding language. A general rule guiding speech in Wolff's opinion is that one should be morally true or honest, that is, say what one believes is true. Yet, Wolff does not take this principle to the supposedly Kantian extreme, in which honesty is more important than anything, even human life. Instead, Wolff clearly states that honesty can never be an excuse for breaking natural law. One should even avoid saying honest things, which would offend someone's feelings. In general, one should not speak frivolously, but one should have always a good reason for saying something.
Wolff also says that no person is obligated to always say the same thing. Indeed, if one doesn't consider anymore as true something that one once held to be true, one need not be accountable for one's earlier opinions. Instead, such a change of opinion is a sign of flexible mind, who can correct oneself when new evidence is found. Yet, there is one particular type of speech that cannot be taken back, namely, promises involving transfer of property or some other activity.
Thus, Wolff's discussion of transference of property and his discussion of honesty are combined in a discussion of pacts or contracts. Just like contract requires more than one person, it cannot be broken just by a one-sided decision, but only by a mutual consensus. If one side of the contract does not do what she has promised, the other side of the contract has the right to force the first person to keep her promise.
Not all contracts are valid, and a valid contract requires something more than mere mutual consent of the persons making the pact. Firstly, the persons making the contract must have enough reason that they are able to make contracts. Thus, minors, people who lack the necessary intellectual capacities and even persons under some severe emotional distress cannot make valid pacts. Secondly, the contract itself must be such that it can be fulfilled. Thus, contracts involving impossibilities or even conditions exceeding the capacities of the persons involved cannot be valid. Hence, no one could have made valid contracts, which would lead them to debts they could never hope to settle.
The majority of the third part of Wolff's natural law goes then into various intricacies of contracts. What makes interest a just part of contracts? That it is a recompensation of potential profits a person could have got by using her capital in another manner. If you have promised something to another person in a letter, can you still take your promise back? As long as the letter hasn't arrived to its destination, you can renounce the promise, but once the person to whom the promise has been made has read and accepted it, the promise becomes a valid contract. Can we in some case presume that a person has wanted to transfer some of his property, although she hasn't said it? If a thing has been derelict for years, we can assume that its owners won't miss it anymore.
Some of Wolff's concerns seem rather quaint these days, such as his long account of oaths, in which person tries to verify what he has spoken and especially to make his promises more believable by insisting that God will curse him if he lies or break his promises. Quite rightly Wolff notes that atheists can't make true oaths, although they can vouch for their own conscience. Furthermore, he notes that such an oath does not really add anything to promises or contracts and it certainly won't make it valid, if it isn't already – thus, although one would vow to do something impossible or beyond one's capacities, one still shouldn't be afraid of hell fire.
Next time we shall see what Darjes had to say about metaphysics.