English: syllogism and enthymeme

The tutor comments on constructions for making assertions.

A syllogism is the familiar structure:

  1. Horses have four legs.
  2. Bessie is a horse.
  3. Bessie must have four legs.

Sometimes, step 1 or 2 is left out. The resulting assertion is an enthymeme:

  1. Susie runs marathons.
  2. Susie must be in good shape.

The missing statement, “Marathon runners have to be in good shape,” is assumed obvious.

Enthymemes are often used in humour:

  1. Joe is a professional student.
  2. He must love Kraft Dinner.

Source:

Hodges, Horner et al. Harbrace Handbook for Canadians. Scarborough: Nelson Education, 2003.

Jack of Oracle Tutoring by Jack and Diane, Campbell River, BC.

English: the difference between “true” and “valid”

The tutor comments on subtleties of logic.

According to Harbrace1, a true statement is one based on widely-accepted, well-supported belief. A valid argument, on the other hand, is based on sound logic. “True” and “valid”, therefore, don’t necessarily apply simultaneously.

The assertion that the infinite series 1+1/2+1/4+1/8+1/16+1/32+….=2 is valid; it can be proven using mathematical logic. Perhaps, however, for it to be true, most people would have to believe it, and be ready to explain why.

That gravity pulls objects to the ground is true, but perhaps difficult to explain or prove with logic.

Perhaps many things we trust are either valid or true, but not necessarily both.

1Hodges, Horner, et al. Harbrace Handbook for Canadians. Scarborough: Nelson Education, 2003.

Jack of Oracle Tutoring by Jack and Diane, Campbell River, BC.