I'm currently reading Sidgewick's The Methods of Ethics, a great deal of which is devoted to arguing the existence of an Ultimate Good that serves as the basis for all philosophical investigation into ethics. Ethical theories that do not, at some level, reference a societal good or an individual good are inherently recursive. To elucidate this point, he critiques Plato's theory of justice, claiming it is circular logic.
>1. It is right to act rationally, and 2. It is right that the Lower parts of our nature should be governed by the Higher...we do not at first feel we are not obtaining valuable information. But when we find that "acting rationally" is merely another phrase for "doing what we see to be right," and again, that the "higher part" of our nature to which the rest are to submit is explained to be Reason, so that "acting temperately" is only "acting rationally" under the condition of special non-rational impulses needing to be resisted, the tautology of our "principles" is obvious.
In other words, it is right to act according to reason, yet acting according to reason is what's right. How would Plato respond?
I don't know about this specific case, but I do know that the Ancients famously have a different conception of how willing, feeling, and thinking relate to one another. It's Christian and post-Christian philosophy (ourselves included) that thinks in terms of "being able to know what is right to do, but not do it." The Ancients thought that a faulty willing was a case of faulty reasoning. Proper knowledge would cause proper behaviour, and improper behaviour was caused by faulty knowledge.
We tend to think that knowing one shouldn't steal is in tension with various desires that urge us constantly to steal. The Ancients would say that if you really, really, really thought about it, and you were fully rational, you would simply not steal. It's a fundamental difference in the conception of human psychology.