According to social contract theorists like Hobbes and Locke, the obligation to obey the state’s laws stems from consent to, yes, a social contract. In today’s readings, Locke describes how that might work while Hume argues that no social contract could possibly be valid.
Locke seeks to address two problems. One is skepticism about the idea that states were formed on the basis of consent, back in the misty dawn of time. We did not read his discussion of that point. The other problem is that it is hard to see how consent could work now. Even if states were originally made by a social contract, there is no opportunity to do that now. When you are born, the state is already running and the current members will not be interested in renegotiating the social contract with each generation as it grows up.
So how is the each generation supposed to consent? Locke’s solution is ingenious. He says that people would agree to a limit on their property: it can be passed on only to those who consent to obey the state. Then, each generation will tacitly consent to obey the state as its members acquire property that is in the state’s “dominion.” Easy!
Of course, as Kamyab noted, this does not work for people who do not have property. To get around that, Locke extends the basic idea of tacit consent to cover everyone who uses anything within the state’s dominion, such as the roads (see §119).
We did not discuss the difference between tacit and express consent at great length. For transactions that require consent, we normally require people to do something that they would do only if they intended to convey their consent such as speaking special words, signing special documents, and so on. This is what Locke calls “express” consent.
“Tacit” consent is given by means of some ordinary behavior as opposed to special behavior. It is controversial because ordinary behavior is an imperfect indicator of an intention to consent. It’s ordinary behavior, after all, that you might have been engaging in for any reason.
That said, for most of us, the suggestion that we have been tricked into consenting to obey the state against our will is far fetched. Most of us just take it for granted that we are obliged to obey the state. If you pressed us, we would probably say something a bit like what Locke says about tacit consent: “well, I live here, use the roads, and so on; why wouldn’t I be obliged to obey the state and its laws?”
We focused on two of Hume’s arguments (Hume  1987, 475). They have the same structure:
Hume’s first argument maintains that consent is valid only if the person giving it believes she has a choice in the matter. In the second argument, he maintained that expressions of consent are valid only if those giving them have a genuine option to refuse their consent which, in this case, means leaving the country.
We spent some time talking about whether having options really is a necessary condition on giving valid consent. We talked about a variety of cases. In some of them, it seemed to us that the absence of a realistic alternative does undermine the validity of an expression of consent. In other cases, it did not seem that way. The trick will be to see which of these cases is more like the state than the others.
One of the cases we discussed comes from none other than David Hume! (In a different book.)
“We may draw the same conclusion concerning the origin of promises, from the force which is supposed to invalidate all contracts, and to free us from their obligation. Such a principle is a proof that promises have no natural obligation, and are mere artificial contrivances for the convenience and advantage of society. If we consider aright of the matter, force is not essentially different from any other motive of hope or fear, which may induce us to engage our word, and lay ourselves under any obligation. A man, dangerously wounded, who promises a competent sum to a surgeon to cure him, would certainly be bound to performance; though the case be not so much different from that of one who promises a sum to a robber, as to produce so great a difference in our sentiments of morality, if these sentiments were not built entirely on public interest and convenience.” (Hume  1995, Book 3, Part 2, Sect. 5, Par. 15)
The relevant part is the sentence about the doctor: “A man, dangerously wounded, who promises a competent sum to a surgeon to cure him, would certainly be bound to performance.” That does not exactly square with his second argument.
The rest of it gives you a taste of something that Hume had said in the chapter on property that we read earlier in the week, namely, that the rules governing promises and contracts are conventional in the same way that the rules governing property are. If so, it’s conventions all the way down and we can dispense with the social contract.
Hume, David. (1748) 1987. “Of the Original Contract.” In Essays, Moral, Political, and Literary, edited by Eugene F. Miller, Revised edition, 466–87. Indianapolis: Liberty Fund.
———. (1740) 1995. A Treatise of Human Nature. Edited by Mark C. Rooks. The Complete Works and Correspondence of David Hume. Charlottesville, VA: InteLex Corporation.
Locke, John. (1680) 1995. Two Treatises of Government. Edited by Mark C. Rooks. The Philosophical Works and Selected Correspondence of John Locke. Charlottesville, VA: InteLex Corporation.