Philosopher David Hume's famous statement.
You can't get ought from is.
Which is to say:
You can't infer 'P ought to be case' from 'P is the case'.
It is not logically valid to say "it is morally Good to do X because it is the case that Y".
This covers a lot of ground; you can't say something is morally right just because most people do it; you can't say that something is morally right just because we evolved that way; you can't say that something is morally right just because it makes the most people happy. For example, we might hold that throwing Christians to the lions is morally wrong -- even if everyone else is doing it, even if we evolved to kill our competitors, and even if it makes the populace happy.
Of course, many people do in effect claim that you can get ought from is -- utilitarians usually base their 'ought' on something akin to measuring happiness, most people are happy to defend things because "we evolved that way", and when in doubt, doing what everyone else is doing is actually a fairly decent heuristic for keeping the peace and helping your fellow humans.
It seems that this is sometimes called Hume's Maxim. I think that 'You can't get ought from is' is correctly called Hume's Law, and that 'one can only believe a testimony to the miraculous if its falsehood would be more miraculous than the fact which it endeavors to establish' is Hume's Maxim. Is this is wrong, or if they are in fact interchangeable, please let me know.