How we change what others think, feel, believe and do
An enthymeme is a syllogism with one part of the argument missing. Thus one or more of the major premise, the minor premise or the conclusion is omitted.
All men are mortal. Socrates is a man. So Socrates is mortal.
Socrates is a man, so Socrates is mortal.
Socrates is mortal.
The first statement is the full syllogism. The second statement has the major premise missing, and the third statement has both the major and minor premises missing, offering just the conclusion. Consequently, it can seem reasonable to omit the two premises as are these should be obvious to most people.
Now consider these:
All Etonians are rich. James is an Etonian. James is rich.
James is an Etonian. James is rich.
James is rich.
In this case, the third statement may be met with the question 'Why? How do you know?'. The second statement could be questioned as to what relationship being Etonian has with James being rich.
Enthymemes are often used in courtrooms:
The gun has the defendant's fingerprints on the trigger. He is clearly guilty!
What is missing here, is the major premise that links the gun to the crime, such as it being found next to the murdered person's body or that bullets from the body match the gun.
Or a famous put down of Dan Quayle:
Senator, I served with Jack Kennedy. I knew Jack Kennedy. Jack Kennedy was a friend of mine. Senator, you're no Jack Kennedy.
When a part of the argument is missing, it is assumed not only to be true, but so obvious that it is not worth including. This makes it very difficult to challenge, as questioning the obvious is an admission of ignorance, which puts oneself lower down the social order and opens oneself to attack. It also uses and 'out of sight, out of mind' principle: when the unsafe part of the argument is missed out, then people may not realize that it has been omitted. Advertisers and politicians thus make great use of enthymemes.