Brandt has developed a theory of ethics based on what it is rational to want - or what he hopes is rational to want. His definition of "rational" involves clear and vivid thinking, complete and accurate information, and no faulty conditioning (e.g. based on atypical events, or having over-generalized stimuli), such that a rational belief (or desire, or whatever) will survive informed criticism by which irrational beliefs or desires would fall.
A number of problems emerge, including the fact that no-one actually knows for certain what is rational in this sense - no one has all relevant information, is free from faulty conditioning, and the rest. Another problem is that, by his system, the superiority of rational beliefs and desires to irrational ones is entirely contingent on our liking them more - and he (perhaps mistakenly) assumes that we do (along the lines of experiencing cognitive dissonance when we find our beliefs or desires to be irrational). He thinks this partly because he thinks all useful desires will be retained - that if a desire generally lead to happiness, it would not be removed by cognitive therapy (which is a suggested method of making ourselves more rational). This belief does not account for the lesson learnt from Schelling's answer to armed robbery, that it can be rational to want to have irrational desires or behaviour... that there can be cases of rational irrationality. [See Reasons and Persons by Derek Parfit, section 5] Say, for example, a religious zealot. He might be brought great happiness by his belief that he is doing God's work. But both his happiness and his desire is based on a belief: his belief in the existence of God. If this belief is irrational, his desire and his happiness from it will not survive cognitive therapy (etc) and he may then be a much more miserable man as a result. Since it is rational (we expect) to want to be happy, the ex-zealot may then want his belief restored... even at the cost of becoming irrational. This kind of situation might be very common - completely rational people may have terrible trouble finding anything to be happy "about" (for as long as anyone needs to be happy "about" anything), and, for them, the appellation "rational" may lose its commendatory force. Being rational is not so hot.