The term "split brain" comes from the world of brain surgery, and refers to a particular surgical technique performed on certain epileptic patients. In order to eliminate the effect of epileptic symptoms spreading through the brain, the bundle of nerves connecting the two halves of the brain—called the corpus callossum—was cut. As an unexpected side-effect, however, split brain patients (who were indeed relieved of their symptoms) were seen to have, as it were, two minds. On the one hand, the dominant hemisphere of the brain, controlling the dominant hand, is linguistic in nature; its approach to the world is rationalistic and abstract. On the other hand, the other, subordinant hemisphere is spatial, and deals with the world in ways that are more emotional, but also more concrete. In normal brains, of course, this division goes mostly unnoticed. However, it seems certain that generally speaking the two halves give us, as it were, two minds, and most people tend to emphasize one side over the other. And this, of course, provides biological support for the dichotomies of psychological types catalogued by Jung.

With this, the rhetor can get on an audience's good side, so to speak. For example, the sciences demand rationality and linguistic rigor, as opposed to the emotional appeal of the arts. Not that scientists don't get emotional, or that artists can't be rational; it's just that the emphasis in these fields lies at different ends of the spectrum. So if you, as a rhetor, ignore these differences and deliver to a group of scientists an argument based on emotion and intuition, you are not as likely to reach your audience as you would with a logical argument. And likewise, if you give a talk to a group of musicians, with the aim of discovering the source of artistic inspiration, but use a method that is deductive and rationalistic, you are apt to come off sounding flat. You must speak to your audience; you must appeal to its nature, to the implicit collective consensus of this "community". Unfortunately (or should I say "fortunately"?), most people are not absolutely one-sided, and most audiences are psychologically diverse. As a result, the best approach is to mix your methods while laying stress upon what you think might predominates in your audience.