The way this was explained to me, and a phrase that I think is very powerful, is that for a contract to be valid there must be a "Meeting of the minds"

(IIRC there must also be two parties (otherwise it is a deed not a contract) and the parties must have "capacity" i.e. the potential to fulfill their obligations)

IANAL - probably painfully obvious to anyone who is.

Law is like code (and magic) - it is a framework that gives real world power to abstract language - maybe this is why HN is so obsessed with legal chitchat