The point is that somebody wouldn't contract without a means with which to enforce the contract -- the whole point of a contract is that everybody has to obey its terms.
Take two people. One owns an orange grove (Person A), and the other has free time (Person B) and is hungry.
Person A comes to Person B and says "If you work in my orange grove today, I will give you 10 oranges." Person B agrees - after all, he's hungry, and he has nothing better to do anyway, and has no reason to doubt Person A.
Person A, though, has a big club, and thus is in the stronger position. At the end of the day, instead of giving Person B 10 oranges, as he promised, A instead pulls out his club and orders B off of his property.
Person B can do nothing about this right now, as Person A has a bigger weapon. All Person B can do is to try to steal the oranges later, find a bigger club, or find some friends, in an attempt to enforce the contract.
But that's not really an enforceable contract - even if Person B succeeds in taking what he was originally promised, all that means is that he was clever, stronger, or found a bigger weapon.
To me, an enforceable contract comes about when there is a body that everyone has agreed to that is able to step in a force Person A to live up to his promise. This doesn't rely on individual interactions in which the stronger person wins - it involves a body which everyone has agreed to (either society as a whole or something within society) arbitrating the matter and enforcing the contract on its merits.
That brings up another problem of contracts in nature - there can be legitimate disagreements over contracts with neither party actually trying to screw over the other. If neither is willing or able to compromise, it again comes down to who is bigger, stronger, or has a bigger club.
However, in a society, society as a whole can agree upon general rules for arbitrating contracts, and things of that nature - they can basically invent contract law. In this case, the parties who disagree on what a contract means are able to take their dispute before a court, and have the problem arbitrated and resolved.
If a state of nature exists if a contract cannot always be properly enforced, then we're still in a state of nature -- people still break laws, and sometimes they get away with it.
It may be punishment enough that nobody's going to make any contracts with the guy in the future since he's known to renege on contracts.
If you mean that laws and punishments must be explicitly denoted in order to be valid,
Contracts are implicitly enforceable,
and by definition people agree to contracts (otherwise they're not contracts).
You seem to be arguing that somebody can be in a state of nature despite being under contractual obligations (which would mean that somebody would not be in a state of total liberty while in a state of nature),
Without an enforcement mechanism, though, their freedom of action is not really limited - they can still do whatever they want, and as long as they are stronger, they will get away with it.
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