Question

Which one of the following does not demonstrate universal instantiation?

a) All robbers wear black and white striped shirts. Therefore, this person who is robbing me is wearing a black and white striped shirt.
b) There do not exist any kittens who are monstrous. Thus, my mom's kitten is not monstrous.
c) There are people with funny hats. So somebody out there must have a funny hat.
d) Nobody is nice. Therefore, Tony is not nice.

Answer 1

C.

Step-by-step explanation:

Universal instantiation: Given a universal generalization (an ∀ sentence), the rule allows you to infer any instance of that generalization. It's a valid rule of inference from a truth about each member of a class of individuals to the truth about a particular individual of that class.

A, B and D have the same premise. From ∀x Cube(x), infer Cube(b). But C, even if it's a logical argument, is not demonstrating universal instantiation. It's using a quantifier for the certain amount of people in that class.