Although some writers propose that scientists should avoid the use of metaphor, others argue that metaphor plays a key role in the process of scientific investigation. The resolution of this dispute requires a precise definition of metaphor and a thorough examination of actual cases of scientific discovery. A behavioral analysis is applied to the surviving examples of Benjamin Franklin’s verbal behavior during his research on lightning. It is concluded that there is no clear evidence that Franklin used metaphoric extension, as opposed to generic extension, in identifying the electrical character of lightning.