"The witness is not here as an expert!" is an objection heard at depositions. The distinction between "fact" witnesses and "expert" witnesses is critical. Fact witnesses are permitted to testify to their sense-impressions. What they saw, heard, felt, tasted or smelled are all fodder for examination, but their predictions on future events or answers to hypothetical questions are not. Experts, on the other hand are there to make predictions on future events, or render "opinions" as they are otherwise called.

The genesis of professional negligence law from biblical times led to the twinning of medical and legal malpractice rules and practice. Legal Malpractice rules arise from the slightly earlier medical malpractice principles. "Continuous representation," is one well-known example. Glamm v. Allen, 57 N.Y.2d 87 (1982); Shumsky v. Eisensein, 96 N.Y.2d 164 (2001). Immunity for strategic choice is another. Rosner v. Paley, 65 N.Y.2d 736 (1985). The absolute need for expert testimony in support of liability against the professional is a third. Sherman v. Ansell, 207 A.D.2d 537 (2d Dept. 1994).

In both medical malpractice and legal malpractice, the defendants are professionals. In both cases, expert testimony on the standard of care and evidence that defendant has departed from that standard is required on plaintiff's prima facie case. These elements may be proven through admissible testimony with different methods of producing that proof. In both, however, only a professional in the same discipline may testify about departures from good and accepted practice.