There is a difference between a reason why something is believed (ratio credentis, an explanation) and a reason why something is true (ratio veritatis, a justification). Ideally the latter would be used for the former, but we do often have reasons, even good reasons, for believing things even if we do not know the reasons why they are true. But if reasons for belief are used as though they are reasons for truth, this has been recognized for most of the history of logic as an informal fallacy , the "genetic fallacy," in which the origin or the cause of a proposition is taken to have some bearing on its truth. It doesn't. The fallacy can take two common forms that are of interest: an ad hominem ("against the man") argument holds something to be false because of where it comes from; and an argument "from authority" (ab auctoritate) holds something to be true because of where it comes from. Both ad hominem arguments and arguments from authority can be very good reasons to believe, or not to believe, something, but they are not logical reasons why something is true. An argument relevant to the truth of its proper subject matter may be called an argumentum ad rem ("concerning the thing," or "to the point").
So someone may believe that racism is evil because their parents told them so, or they may believe that there are billions and billions of galaxies in the universe because they heard Carl Sagan say so, and these may well be good reasons to believe these things (just as it is reasonable to say that Ptolemy believed in geocentrism because he lived in the 2nd century AD); but they are not reasons which logically bear on the truth of the propositions. We think of Carl Sagan as a worthy "authority" because he is a generally respected professional astronomer, and we expect that astronomers are the ones who have concerned themselves to know the reasons which do bear on the truth of the statements they make. If we were interested in taking the time, and if we had the ability to master the discipline of knowledge, we suppose that we could become familiar with the reasons for truth of professional astronomers' statements. The galaxies, or the moons of Jupiter, are there for us, or anyone, to have a look at, with the right equipment and the right knowledge to understand what we are seeing. Now when it comes to the evil of racism, it may be less clear what the reasons bearing on the truth of the proposition may be; but it is equally clear that genetic reasons do not bear on truth any more than they do in astronomy -- the genetic fallacy does not depend on the content of some particular area of discourse but is a general principle of logic. And so if someone says that cultural, psychological, and historical reasons are "ways of justifying belief," they are playing on an ambiguity, since they may well justify belief, but they cannot justify truth; and if someone thinks so, they simply make a logical error in reasoning. 
For the sake of argument, let's suppose that there are no reasons for the truth of ethical propositions. That is what Hume thought, and strangely enough he was not a relativist -- he thought we believed in all kinds of things (like the necessity of cause and effect) whose truth reason was unable to establish. Such a state of affairs would not change the logical status of genetic reasons, it merely puts Hume in the same camp as Pascal, that the "heart has reasons the mind cannot understand." The question then is whether ethical statements would be less true in some sense than scientific statements. If Hume himself was right, and all scientific statements depend on the causal principle, which is known in no different a fashion than the principles of moral sentiment, then certainly ethical statements would not be any less true than the scientific. Hume didn't see any difference -- he saw morals as the science of human nature as other sciences were directed towards external nature. And he felt he had achieved as much certainty in morals as Newton had in physics.
There is an excellent argument for free will based on the distinction between reasons for truth and reasons for belief; for the determinist must admit that, if determinism is true, then the determinist believes in determinism because that belief is caused, like everything else in the mind. And causes, of course, are often, or usually, hidden from our examination. So the Freudian can say, "You only believe that because you hated your father," or the Marxist can say, "You only believe that because you are a bourgeois capitalist exploiter of workers." But if the belief of the determinist in determinism is determined by causes, then that belief provides no evidence whatsoever for the truth of determinism. We can be caused to believe false things. Determinists really can only argue that they know that determinism is true, i.e. have reasons for truth, if they admit that determinism is wrong in that their beliefs are not determined by causes alone. Truth requires that we be able to examine reasons as reasons for truth, without such reasons (whether they are reasons for truth or not) compelling our belief.
i.e. it does not violate the formal rules of deductive logic.
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This distinction lies behind the entire Problem of Knowledge; for Descartes realized that if perceptions are caused, then it is a genetic fallacy to suppose that perceptual knowledge is true just because perceptions occur. They could have been caused by brain fever, drugs, or the Deceiving Demon. His answer is actually an argument from authority -- the authority of a non-deceiving God. Answering a genetic fallacy with another genetic fallacy may be one reason why his answer has not historically been very satisfying.
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