Enthymeme is an informally-stated syllogism which omits either one of the premises or the conclusion. The omitted part must be clearly understood by the reader. The usual form of this logical shorthand omits the major premise:
- Since your application was submitted before April 10th, it will be considered. [Omitted premise: All applications submitted before April 10 will be considered.]
- He is an American citizen, so he is entitled to due process. [All American citizens are entitled to due process.]
An enthymeme can also be written by omitting the minor premise:
- Ed is allergic to foods containing monosodium glutamate, so he cannot eat Chinese food seasoned with it.
- A political system can be just only when those who make its laws keep well informed about the subject and effect of those laws. This is why our system is in danger of growing unjust.
It is also possible to omit the conclusion to form an enthymeme, when the two premises clearly point to it:
- If, as Anatole France said, "It is human nature to think wisely and act foolishly," then I must propose that the Board of Supervisors in this case is demonstrating human nature perfectly well.
- The Fenton Lumber Company never undertakes a clearcut until at least eighty percent of the trees are mature, and the 4800-acre stand of pine above Mill Creek will not be that mature for another fifteen years.
Whenever a premise is omitted in an enthymeme (and understood by the reader), it is assumed to be either a truism or an acceptable and non-controversial generalization. But sometimes the omitted premise is one with which the reader would not agree, and the enthymeme then becomes a logical fallacy-an unacceptable enthymeme. What are the omitted premises here, and why are they unacceptable?
- You can tell this tape recorder is a bunch of junk: it's made in Japan.
- He says he believes that Jesus was a great moral teacher, so he must be a Christian.
- Those kids are from Southern California? Then they must be either crazy or perverted.
It goes without saying that you should be careful in your own writing not to use enthymemes dishonestly--that is, not to use clearly controversial assertions for the omitted premises.
Aside from its everyday use as a logical shorthand, enthymeme finds its greatest use in writing as an instrument for slightly understating yet clearly pointing out some assertion, often in the form of omitted conclusion. By making the reader work out the syllogism for himself, you impress the conclusion upon him, yet in a way gentler than if you spelled it out in so many words:
- It is essential to anchor the dam in genuine solid rock, rather than in sandstone, and the Trapper's Bluff area provides the only solid rock for seven miles on either side of the designated optimum site.
- Yes, it is a beautiful car, but it does not have an automatic hood-ornament washer, and I just will not have a car without one.
