The Duality of Practical Reason, or How to Butter Bread

Henry Sidgwick developed a moral argument for the existence of something like a god in The Methods of Ethics, Book IV, Chapter VI:
1. What I have most reason to do is always what will best secure my own happiness in the long run.
2. What I have most reason to do is always what morality requires.
3. If there is no moral governance of the universe, what will best secure my own happiness is not always what morality requires.

While Sidgwick accepted the argument as valid, he refused to accept the conclusion, namely, that there must be just such a governance. He summarized his reason for rejecting the conclusion by stating:

The fact that I cannot act rationally without assuming a certain proposition, does not appear to me...a sufficient grounds for it being true.

The above argument is examined at length by J.L. Mackie in The Miracle of Theism in a chapter focusing on dismantling morality arguments for the existence of God(s). The duality is profound, however, and more interesting consequences about the epistemic status of the propositions can be found here.

Mackie's view aligns with our discussion during the most recent club meeting that the validity of the premises only warrant belief in the conclusion but do not indicate anything about reality. This seems to mirror the structure-based description analyzed in modal actual/possible terms. Mackie gives a comparable argument to elaborate:

1. If the enemy are advancing in overwhelming strength, then, if we do not withdrawal, our army will be wiped out;
2. We must not allow our army to wiped out;
3. We must not withdraw, because that would mean letting down our allies;
Conclusion: The enemy is not advancing in overwhelming strength

If one allows the syllogistic structure then one might be pressed to think this argument is sound, but in all such cases we cannot take what we are "inclined to think that is it rational to do" as evidence about the facts of the world. That is, what is rational to do depends on the facts, not the other way around.