http://en.wikipedia.org/wiki/Appeal_to_authority
An appeal to authority or argument by authority is a type of argument in logic. It bases the truth value of an assertion on the authority, knowledge, expertise, or position of the source asserting it. It is also known as argument from authority, argumentum ad verecundiam
It is one method of obtaining propositional knowledge, but a fallacy in regard to syllogistic logic, because the validity of a syllogism is independent of the qualities of the source putting it forward. The converse case is an ad hominem attack: to imply that a claim is false because the asserter lacks authority or is otherwise objectionable in some way.
On the other hand, arguments from authority are an important part of informal logic. Since we cannot have detailed knowledge of a great many topics, we must often rely on the judgments of those who do. 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 source 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.
“They must find it difficult, those that take authority as truth, instead of truth as the authority”…