According to him, we have been quite mistaken in thinking that matters of fact and matters of value are two separate things. David Hume was wrong when he said we can never legitimately go from the way things are to the way things ought to be. G. E. Moore made a mistake when he invented the “naturalistic fallacy” for moves from the scientific to the ethical.
Unfortunately, Ruse is wrong about Hume here: Hume doesn't say that we can never legitimately go from the way things are to to the way things ought to be; he says that in all the standard systems of morality he's come across, the author starts out with 'is' propositions and then gets to 'ought' propositions without explaining this new relation, which needs to be explained; and he says that this shows that 'ought' is not to be treated as a relation perceived by reason alone. Hume never claims that we can "never" go from 'is' to 'ought'; he claims that in rationalist and popular systems of morality, which treat 'ought' as a relation, this transition is never explained properly, and that this is reason to think that the transition is impossible if 'ought' is treated as a relation.
It's also somewhat misleading to bring up Moore's account of the naturalistic fallacy here without making clear that the majority of philosophers have accounts of ethics that are inconsistent with it: utilitarianism, to give just one instance, is exactly the sort of thing that Moore thinks commits the naturalistic fallacy, and lots of philosophers are utilitarians and so hold views that commit them to saying that Moore's account is, at the very least, flawed or misleading. Moore's account doesn't eliminate "moves from the scientific to the ethical"; it eliminates any definition of 'good' in terms of 'natural quality' (Moore never seems to have come up with a reasonable account of what counts as a 'natural quality'). The moves are fine; they just aren't definitive of good. And if Moore's account did eliminate such moves, it would not eliminate just scientific ones, but (apparently) any connection of ethical domains with any sort of natural domain, scientific or not (as Prior pointed out once).