Maggie's Farm

The opinions of others may be interesting as showing the trend of public sentiment, but they add nothing to proof.

There is one kind of argument that forms an exception to this rule--the so-called argument from authority. It has very small place in ordinary discussions. The argument from authority is the use of testimony from a witness of such eminence and unquestioned impartiality that his word carries conviction to all. There are few questions commonly argued in which this kind of evidence as to matters of opinion could be needed in the proof: for the point sought to be established would usually be a point admitted in advance, and not among the issues as all. Expert testimony of the ordinary sort offered in the courts is very far from being entitled to claim such authority. For every expert witness on one side another expert can usually be found on the opposite side; and this is true not only of lawsuits and criminal trials, but also of any ordinary question involving technical matters. Expert witnesses are good witnesses so long as they confine themselves to facts--provided they can be shown to be reasonably impartial; but when they begin to state opinions, such testimony proves nothing more than that the people who know the most about the subject disagree--which fact we knew already. Little attention need be given in most arguments to testimony as to opinions without facts on which the opinions are based. the sort of cases in which such evidence is valid is, for example, the opinion of a college president as to the meaning of a college rule; the appeal to scriptures for principles of right and wrong; the constitutional decisions of Chief Justice Marshall. These are not the kind of questions that are likely to be at issue in ordinary discussion. With this one exception, all evidence directed to the establishment of facts, not to the question of opinions. For support of our opinions, we use the facts proved by evidence as interpreted by what is called reasoning.