On the other hand, there is no fallacy involved in simply arguing that the assertion made by an authority is true, in contrast to claiming that the authority is infallible in principle and can hence be exempted from criticism: It can be true, the truth can merely not be proven, or made probable by attributing it to the authority, and the assumption that the assertion was true might be subject to criticism and turn out to have actually been wrong. If a criticism appears that contradicts the authority's statement, then merely the fact that the statement originated from the authority is not an argument for ignoring the criticism.
The first form of the appeal to authority is when a person presenting a position on a subject mentions some authority who also holds that position, but who is not actually an authority in that area. For instance, the statement "Arthur C. Clarke released a report showing it is necessary to floss three times daily" should not convince many people of anything about flossing, as Clarke, a science fiction writer, was not a known expert on dental care. Much advertising relies on this logical fallacy in the form of endorsements and sponsorships. A sportsperson or actor, for example, is no more likely than average to have an specialist knowledge of watches or perfume, but their endorsement of a particular brand of watch or perfume is very valuable in advertising terms. In some cases, the advertisers use an actor's well-known role to imply that the person has authority in an area; an actor who plays a doctor on television may appear in their white coat, and endorse a drug or health product.
The second form, citing a person who is actually an authority in the relevant field, carries more subjective, cognitive weight. A person who is recognized as an expert authority often has greater experience and knowledge of their field than the average person, so their opinion is more likely than average to be correct. In practical subjects such as car repair, an experienced mechanic who knows how to fix a certain car will be trusted to a greater degree than someone who is not an expert in car repair. There are many cases where one must rely on an expert, and cannot be reasonably expected to have the same experience, knowledge and skill that that person has. Many trust a surgeon without ever needing to know all the details about surgery themselves. Nevertheless, experts can still be mistaken and their expertise does not always guarantee that their arguments are valid.
In some cases, the appeal to authority plays on the Western culture's respect for credentials. For example, suppose a complex nutritional system and diet guide is endorsed or ghostwritten and credited to a qualified doctor. While a doctor does receive general training on nutrition and diet, they may not be an expert on nutrition and diet, a field for which an expert will often possess PhDs in nutrition and certification as a dietician. The same technique is used with the PhD degree; an advertiser may reinforce their claims about a product by appending an endorsement from John Doe, PhD, but without stating what area the PhD is in. If the product being endorsed is foot powder, and Dr. Doe studied podiatry, the endorsement carries some weight, but if he studied film criticism, he may have no more than average knowledge of the product and its merits.
In mathematics, the second form, especially when the appellant is himself the authority, is wryly referred to as "proof by tenure".
The first statement is called a 'factual claim' and is the pivot point of much debate. The last statement is referred to as an 'inferential claim' and represents the reasoning process. There are two types of inferential claim, explicit and implicit.
Arguments that (fallaciously) rely on the objectionable aspects of the person for the truth (usually falsity) of the conclusion are described as ad hominem arguments.
The fact that an argument is an appeal to authority does not make its conclusion untrue, nor does it make it unreasonable to believe the argument. An appeal to authority cannot guarantee the truth of the conclusion (see Truth and Consensus theory of truth), because the fact that an authority says something does not make it so. Ideally, propositions being true (or having arguments supporting them) is what makes authorities believe them to be true, not the other way around. An appeal to authority thus confuses cause and effect. Furthermore, notice that a rigorous concept of truth is a complex subject.
Martyr fallacy — The loser is axiomatically correct by virtue of failure (antithesis of Last Man Standing Fallacy), also known as the Underdog Fallacy.
Not universal taxonomy [nomenclature derived from "Lectures on Logic" Dioc.North [Paul Priest, 1991]