Unenforceable contracts and contracts in the state of nature
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Some contracts made in the state of nature are void; made "in a civil estate," good.
If a covenant be made wherein neither of the parties perform presently, but trust one another, in the condition of mere nature (which is a condition of war of every man against every man) upon any reasonable suspicion, it is void: but if there be a common power set over them both, with right and force sufficient to compel performance, it is not void. For he that performeth first has no assurance the other will perform after, because the bonds of words are too weak to bridle men's ambition, avarice, anger, and other passions, without the fear of some coercive power; which in the condition of mere nature, where all men are equal, and judges of the justness of their own fears, cannot possibly be supposed. And therefore he which performeth first does but betray himself to his enemy, contrary to the right he can never abandon of defending his life and means of living.
But in a civil estate, where there a power set up to constrain those that would otherwise violate their faith, that fear is no more reasonable; and for that cause, he which by the covenant is to perform first is obliged so to do.
Hobbes, Lev XIV 18-19
Contracts made out of fear in the state of nature are valid.
Covenants entered into by fear, in the condition of mere nature, are obligatory. For example, if I covenant to pay a ransom, or service for my life, to an enemy, I am bound by it. For it is a contract, wherein one receiveth the benefit of life; the other is to receive money, or service for it, and consequently, where no other law (as in the condition of mere nature) forbiddeth the performance, the covenant is valid. Therefore prisoners of war, if trusted with the payment of their ransom, are obliged to pay it: and if a weaker prince make a disadvantageous peace with a stronger, for fear, he is bound to keep it; unless (as hath been said before) there ariseth some new and just cause of fear to renew the war. And even in Commonwealths, if I be forced to redeem myself from a thief by promising him money, I am bound to pay it, till the civil law discharge me. For whatsoever I may lawfully do without obligation, the same I may lawfully covenant to do through fear: and what I lawfully covenant, I cannot lawfully break.
Hobbes, Lev XIV 27
Promises impossible to perform are void.
And therefore, to promise that which is known to be impossible is no covenant. But if that prove impossible afterwards, which before was thought possible, the covenant is valid and bindeth, though not to the thing itself, yet to the value; or, if that also be impossible, to the unfeigned endeavour of performing as much as is possible, for to more no man can be obliged.
Hobbes, Lev XIV 25
The only source of the force of contracts in the state of nature is the fear of God.
The force of words being (as I have formerly noted) too weak to hold men to the performance of their covenants, there are in man's nature but two imaginable helps to strengthen it. And those are either a fear of the consequence of breaking their word, or a glory or pride in appearing not to need to break it. This latter is a generosity too rarely found to be presumed on, especially in the pursuers of wealth, command, or sensual pleasure, which are the greatest part of mankind. The passion to be reckoned upon is fear; whereof there be two very general objects: one, the power of spirits invisible; the other, the power of those men they shall therein offend. Of these two, though the former be the greater power, yet the fear of the latter is commonly the greater fear. The fear of the former is in every man his own religion, which hath place in the nature of man before civil society. The latter hath not so; at least not place enough to keep men to their promises, because in the condition of mere nature, the inequality of power is not discerned, but by the event of battle. So that before the time of civil society, or in the interruption thereof by war, there is nothing can strengthen a covenant of peace agreed on against the temptations of avarice, ambition, lust, or other strong desire, but the fear of that invisible power which they every one worship as God, and fear as a revenger of their perfidy. All therefore that can be done between two men not subject to civil power is to put one another to swear by the God he feareth: which swearing, or oath, is a form of speech, added to a promise, by which he that promiseth signifieth that unless he perform he renounceth the mercy of his God, or calleth to him for vengeance on himself.
Hobbes, Lev XIV 31