The deontic fallacy is otherwise known as the "normative" or "moral" or "Naturalistic" fallacy, and is committed when one presents an argument in which the premises are descriptive statements (asserting that something is the case) but where the conclusion is a prescriptive statement (asserting that something ought to be the case). There is many times a hidden premise in an argument that commits the deontic fallacy, and this hidden premise is always a value statement (asserting a value one holds). For example,

Black South Africans have been seen by White Afrikaners as second-class citizens and nonpersons for hundreds of years. This is an outgrowth of the way South Africa grew, historically, as a nation. Therefore, this is the way it should be.

The hidden premise here is that history always works out for the best, an assertion of value. This, of course, is debatable, as are all value judgments we encounter. One can attack this argument from at least two sides: 1) by attempting to show that the hidden premise is false, and/or 2) by attempting to show that the conclusion itself is false. Another way of showing its unacceptability is to present a counterexample, such as, Jews have been considered second-class citizens and nonpersons for hundreds of years. The Third Reich's treatment of Jews was an historical outgrowth of this attitude toward the Jews. We conclude that this treatment was the way it ought to have been.

One of the distinct problems with this argument as a counter-example to the argument above is that it also relies on a value judgment, one perhaps not shared by all readers of this text. The most that can be said here is that the argument itself is simply formally invalid, which means that the conclusion does not follow from the premises.

The deontic fallacy, being one of the most common of all fallacies, is many times persuasive. The primary reason for this is that the hidden premise is many times one we would presumably accept. Consider:

Smoking cigarettes causes shortness of breath, diminishment of circulation, heart disease, bronchial infections, and lung cancer. Hence, one ought not smoke.

The hidden premise here is that one ought to avoid all of these effects of cigarette smoking. This is a case in which, if the hidden premise is added to the argument, the argument becomes more or less acceptable. However, it is possible that the hidden premise might be rejected by a person who could present an acceptable argument for doing so, say, in a situation where a number of people had agreed (having been informed of the possible consequences) to smoke cigarettes for a research project studying the effects of cigarette smoking on distance runners. The point is, again, that value judgments are open to question as to their truth. They are not simply statements of objective fact (whatever that is). That is, a value statement, such as, "New York is as beautiful as New Orleans", is not in the same category with such sentences as, "Chicago is further from San Diego than Detroit is from Toronto", in terms of discovering their truth.

When there is differing opinion about the truth value of a sentence in an argument, the acceptability of the argument is also in question. To get at the disagreement usually takes getting clear on what values are being pulled in to the arena of judgment about the argument. When the values themselves are made explicit, further new arguments will result, having to do with the acceptability or unacceptability of these values, and so on. It is only when agreement is reached on these values that the original argument itself can be agreed upon as acceptable or the reverse.

One of our jobs here is to try to discover when an argument is valid and when not. The fact that one cannot validly go from a fact to a value indicates that any argument containing only factual statements in the premises and a value statement in the conclusion is not valid, no matter whether one thinks it acceptable or not. Thus, for example, many people have argued that,

Lying, in any form, creates distrust. Literally no one wants to be distrusted. Therefore, one ought never lie.

I think it cannot be denied that anyone caught in a lie is usually thereafter, to some extent, distrusted. And it is probably true that the vast majority of people would prefer to be trusted than distrusted. Does it follow that one ought never lie? It does not, since perhaps there are cases in which one might attempt to show it better (a value term) to lie, be found out, and be distrusted than tell the truth and avoid the risk of being distrusted. Just from the practical point of view, it seems the above argument cannot be defended against all situations. But, further, those who heartily agree with the idea that one cannot derive an 'ought' from an 'is' ask, What could it be in the premises that might possibly lead anyone to think the conclusion follows? So smoking is bad for one's health. Does it logically follow that one ought to stop? So polluting the environment will kill seventy-five percent of the wildlife. Does it logically follow that we ought not pollute? The strictly logical answer to these questions is "no". So that black widow spider is crawling up the nape of your neck. Does it logically follow that you should try to remove it? No! But, behind all these "oughts" are values which influence our accepting the inferences we do accept. From the point of view of logic alone, the arguments are not valid, but from the view of one who holds the various values indicated, there is no hesitation to accept the argument. The moral here is that one must take care to make one's values explicit, so there is no confusion about the "hidden" values present in any argument that might be charged with committing the deontic fallacy.