"THE RHETORICAL BURDEN OF EXPERT WITNESSES"

Under what conditions is it legitimate to accept some claim on the basis of authority—in particular, the authority of an expert? Let the claim be p. A widespread textbook view is that for it to be legitimate to accept p on the basis of the authority of an expert, E, the following conditions must be satisfied: (1) p must belong to some body of knowledge, k; (2) E must be an expert on k; (3) E must not have a vested interest in p's being true, or be otherwise suspect as a professed authority on whether p is true; and, finally, (4) the experts on k must agree about p. (Cf. Govier: 84.) This last condition—call it the agreement condition—is of special relevance to the authors ' concerns. We find the condition affirmed in textbooks, in various ways. For example, one textbook says that we assess the credibility of an expert's testimony partly by seeing whether or not the matter is controversial among experts. If the experts disagree about some claim, then we can't decide for or against that claim on the basis of what the experts say. (Pinto, Blair, Parr: 34-5). According to another textbook, it is legitimate to appeal to authorities speaking in their fields, "unless the issue is a controversial one in the field, one over which the experts disagree " (Freeman: 58). For an appeal to authority to be legitimate, there must be "general consensus " in the field in question concerning the claims endorsed (ibid.: 158)