This is a very puzzling statement. To me, it appears as wildly absurd implausible for anyone to even say such a thing, as absurd as saying "P, but I don't believe that P." [Moore's Paradox] If you're not going to do X, what precisely do you mean by "should"?
But unlike Moore's Paradox, people actually do say, "I shouldn't do X, but I will do X." What can they possibly mean? People seem say it sincerely, and it seems intuitively to make some sort of sense for me; I can't conclude such people regularly become suddenly insane and start speaking gibberish.
Of course the speaker might just be bullshitting the listener. "[You expect me to say] I shouldn't do X [but I myself don't believe it] but I [therefore] will do X." I suppose such a speaker prefers to be thought a hypocrite or weakling rather than a libertine.
But there are more sincere instances. In the movie Happiness, for example, one of the characters, a homosexual pedophile, "breaks down" and rapes two ten year-old boys (his son's friends). You can see the wheels turning in his head: "I shouldn't rape these boys, but I will." He's not literally insane, not in any sort of unconscious "fugue"; he consciously makes complex preparations. But he's so quickly arrested that he must have realized he would be. He's not bullshitting anyone else; he doesn't try to justify his behavior, and if he himself merely believed that society didn't want him to rape those boys, he wouldn't have so easily allowed himself to be caught. Clearly he's absolutely sincere about both clauses of the Paradox of Motivation.
To anyone who holds "outer-directed" ethics, whether theistic ethics or non-theistic moral objectivism, is going to have trouble with the Paradox of Motivation. By explicitly decoupling ethics from (subjective, "inner-directed") motivation, they must answer the question: what motivates a person to do what they believe is good?
There are two alternatives: Either the evaluation of what is good is itself a motivation, or the evaluation of what is good is not itself a motivation. In the first case, we're talking explicitly about meta-ethical subjective relativism: A person does X because they evaluate X to be good. In the second case, though, a person will, by definition, do what they are motivated to do (i.e. a "motive" is defined to be that brain structure which causes action), and it's pointless to discuss how to evaluate the good; we are better off discussing what motivates people, and we're back to meta-ethical subjective relativism.
If you do what you believe to be good because you want to do what's "good", then doing good different is doing what you want. And if you don't want to do good, why are you doing it? If there's some extrinsic magic causal force that makes you do good, regardless of what you want, then how is doing good any more of a choice than "obeying" the law of gravity?
We can see the same sort of thing in Micah Cowan's answers to my questions:
Obviously, if it was good, then it was what God wanted me to do. And, likewise, if it was something God wanted me to do, then it was good. ...But Micah isn't explaining his motivation, he's just restating it. He does things because he loves God, but "loves God" is just a way of saying that he does things because he thinks God wants him to do it. But he also things that God wants him to do it if he already thinks it's good. He's saying, in effect that at least for some things, he's doing what he thinks is good; some of his religious belief are the effects of his beliefs about the good, not the cause.
The thing is, I really loved God. That's an understatement. I was absolutely, 100% in love with God (not in the South Park sense ;) ). I would go well out of my way to do something if I "knew" it was what God wanted.
But we can reasonably conclude that Micah's religious beliefs entailed that he would do some things that he determined extrinsically (e.g. by reading the Bible or listening to his clergy-critter) that God wanted him to do. He wants to do good, and it's good to do what God wants, and there are some things he concludes that God wants other than the totality of his innate desires. In this case, his beliefs about God have not supplied the motivation (i.e. he still wants to do what he thinks is good); rather, his religious belief have altered his evaluation of what is good. And we can conclude that his religious beliefs have altered his evaluation only to the extent that his beliefs have changed now that he's an atheist.
Micah cannot say that he wanted to do good because God wanted him to do good. "God wants me to do X" is, for some of his beliefs, just an elaborate way of saying, "I myself want to do X." We can conclude that his religious beliefs motivated him to do only those things which, as an atheist, he no longer believes to be good. Therefore, we can conclude that his religious belief did not motivate him at all to be good, at least by his own evaluation. Indeed, we can conclude that religious belief never motivates a person to do that which he would consider good if he did not believe in God; it can motivate him — only indirectly by changing what he believes to be good — only to do that which he would not consider good if he did not believe.
It is, of course, obviously the case that religion can motivate someone else to do that which I myself consider to be good. But my perspective is not privileged (except to myself).
In the next post, I'll talk about the Paradox of Motivation directly from the perspective of meta-ethical subjective relativism.
Also, there is one sense in which religion can in some sense "motivate" doing good, which I'll discuss in the following post.
Update: Fixed the title and the closing.