Existential Instantiation and Existential Generalization are two rules of inference in predicate logic for converting between existential statements and universal statements.

Universal Generalization is a rule of predicate logic that lets you go from a statement about an individual to a generalization, but there are restrictions on how it can be used.

For the past 14 lessons, we have been focused on sentential logic. This is where we use single letters in place of whole sentences and focus on the logical relations made between whole claims. But an argument can be valid without there being logical relations between whole sentences.

Indirect proof, or reductio ad absurdum, is a method for proving something by showing that the assumption of its opposite would entail a contradiction.

Conditional proof is a method for proving a conditional by assuming its antecedent and deriving its consequent. Read this post or watch the video to learn how it’s done.