Hume said you can never draw conclusions from is sentences to ought sentences.
Highly disputable, given that many acts of morality are hereditary, and can be observed in other species, certainly not all, but at least for us, it has proven to be a successful evolutionary strategy. (but certainly it wasn't known in Hume's time)
Another argument is from probability, the more people do good and expect to receive good in return, the more good there will be in the world.
