The so-called Rabbit Principle is one of the simplest, and yet most profound and useful of all rules of good reasoning. Informally, it is:

    Rabbit Principle: contentions should contain no magic rabbits.

More prosaically:

    Every significant term or phrase appearing in the conclusion of a simple

    argument should also appear in a premise of that argument.

Figure 2.12

For example, consider the VitaChaff argument above. Note that the contention contains the following significant terms or phrases:

  - VitaChaff

  - is good for you

Notice also that each of these appears somewhere in the premises; indeed, in this case each one appears in a distinct premise. This argument observes the Rabbit Principle.

Figure 2.13

This argument is much more typical of peoples’ ordinary reasoning. The contention contains the following terms or phrases:

  - Humanity

  - faces

  - a period

  - unprecedented

  - turmoil

Not a single one of these appears in the premise as provided. The terms are magic rabbits; they suddenly appear ‘out of nowhere’ in the contention. In this argument there is a big gap, or ‘leap of logic’, between the premise and the contention.

Why ‘Rabbit’?

The name Rabbit Principle alludes, of course, to that most clichéd of all magician’s tricks, pulling a rabbit out of a hat. However things appear on stage, we all know that if a rabbit is pulled out of a hat, it must have already been in the hat; rabbits do not appear ‘by magic’. Similarly, if you want to prove something about Humanity, then you will have to say something about it in your premises.

Why Care about Rabbit Principle Violations?

Observing the Rabbit Principle has a number of salutary effects on our reasoning, including:

  - helping us articulate fully the premises needed to establish the contention,

  - helping us tighten the connection between premises and contention.

In short, observing the principle makes reasoning more explicit and rigorous.

The Rabbit Challenge

The Rabbit Principle is a very simple concept, but observing it fully can be surprisingly challenging. First, it is much easier than you might think to miss a magic rabbit; the eye glides over the text, often failing to notice the sudden irruption of new ideas.

Second, telling what is, and what is not, a violation of the principle often requires subtle judgment. Is ‘Humanity’ the same thing as, say, ‘billions of people’? Is the word ‘is’ significant here, for the purposes of the principle? Third, making the right adjustments in the contention, or the premises, to fix Rabbit violations can present quite a puzzle.

Fortunately, as with most skills, one gets better – and faster – with practice.