Quasi-syllogism is a categorical syllogism where one of the premises is singular, and thus not a categorical statement.
For example:
- All men are mortal
- Socrates is a man
- 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:
- Socrates is a man
- 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.