...we may observe, that it is impossible for men so much as to murder each other without statutes, and maxims, and an idea of justice and honour. War has its laws as well as peace; and even that sportive kind of war, carried on among wrestlers, boxers, cudgel-players, gladiators, is regulated by fixed principles. Common interest and utility beget infallibly a standard of right and wrong among the parties concerned.
David Hume, An Enquiry Concerning the Principles of Morals, Section IV. Some of the points Hume adduces for this conclusion are amusing, although nonetheless on point; I particularly like this one:
I hate a drinking companion, says the Greek proverb, who never forgets. The follies of the last debauch should be buried in eternal oblivion, in order to give full scope to the follies of the next.
Or, in other words, human beings will come up with rules for distinguishing right and wrong even if it is a matter of rules distinguishing the right and wrong ways to debauch themselves.