Quasi-syllogism is a categorical syllogism where one of the premises is singular, and thus not a categorical statement.

For example:

  1. All men are mortal
  2. Socrates is a man
  3. Socrates is mortal

In the above argument, while premise 1 is a categorical, premise 2 is a singular statement referring to one individual. While this is a valid logical form, it is not strictly a categorical syllogism.

Of course, it has been suggested that you can translate any singular statement into a categorical.

For example:

  1. Socrates is a man
  2. All members of a class of which the only member is Socrates are men

The above two premises may be considered identical, but the first is a singular and the second is a categorical.

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