Kant points out that “ought implies can”. That is, to say that we ought to seek the highest good is to say that there is a highest good to be sought. If this is true, then one is left fleshing out what the “highest good” is, and will end up with something very much like (if not exactly like) God.
I’ll not get into the details as to why the highest good is probably God. Mackie doesn’t object to this, and I’ve never heard anyone else do so, either. Still, he does pick an odd spot. He agrees that we ought to seek the highest good, but claims that this only implies that we can seek it, not that it is actually there to be found.
I think Mackie has essentially missed the point here. Whether or not “we ought to seek” means that we “can find”, it clearly means that there is a good to be found. Else, there is simply no basis of “ought” to begin with.
Of course, more popular objections to these kinds of arguments simply affirm relativism and claim that is the end of the argument. One doesn’t see this out of Mackie, I suspect, because relativism is far less popular among the philosophically sophisticated than it is in popular culture. As well-loved as relativism is in dorm rooms and among pot-smokers, apparently it is much less so in the professors’ lounge.
Still, if one wants to deny moral realism, one can do so. The real problem with this isn’t what professors think. It is that 1) the view is unlivable and 2) it rests on philosophical presuppositions which are themselves highly questionable.
As I’ve already discussed the second issue, I’ll not repeat my thoughts on it here. But, regarding the first, I think this is a bigger point than most non-theists seem to think. At the very least, it deserves more than a shrug and a “that doesn’t prove that it is false”.
This is because, if the atheist is going to say that no one can actually live in a rational way, then he has abandoned the main line of attack leveled at theists–that theism is an irrational approach to life. Thus, to behave as if morality exists while embracing relativism or nihilism is to agree that one has no rational advantage over the theist.
And this is on the committed atheist’s own view–if one happens to see any value in any of the arguments for theism, rationality is that much in favor of the theist.