"The economic security of Bitcoin and other proof-of-work cryptocurrencies relies on how expensive it is to rewrite the blockchain. If a 51% attack were economically feasible, an attacker could send a transaction to a victim, launch the attack, and then double spend the same coins back to themselves. Satoshi Nakamoto assumed that this would not occur because a majority of miners would find it more lucrative to honestly follow the protocol than to attack the chain, the source of their own mining revenues."