What you’re really asking, I think, is, “Why should I think that objective moral values exist rather than that evolution has made me believe in the illusion that there are objective moral values?” And the answer to that question is, “Because I clearly apprehend objective moral values and have no good reason to deny what I clearly perceive.”
This is the same answer we give to the sceptic who says, “How do you know you’re not just a body lying in the Matrix and that all that you see and experience is an illusory, virtual reality?” We have no way to get outside our five senses and prove that they’re veridical. Rather I clearly apprehend a world of people and trees and houses about me, and I have no good reason to doubt what I clearly perceive. Sure, it’s possible that I’m a body in the Matrix. But possibilities come cheap. The mere possibility provides no warrant for denying what I clearly grasp.It's weird to hear Craig say this, in light of his response to an unrelated question. When pressed on why the observation of physical causality within the universe should warrant us to infer causality applying to the universe, Craig replied:
Ask [atheists] why one timeless entity—say, a number—could not depend timelessly for its existence on another timeless entity. Why is that impossible? Why couldn't God timelessly sustain a number in existence? That would clearly be an asymmetric causal relation. Why is that impossible?Craig seems perfectly willing to retreat to the merely possible when he's arguing for something that seems intuitively true or obvious to him, but when he's less credulous he is quick to point out the folly of that kind of thinking. I believe this is what Richard Dawkins would call "intellectual compartmentalization".