Symbolic Logic: Conditional/Indirect Proofs and Proving Theorems

Hey guys! So, this time, we’re going look at other methods we can use to construct proofs when just deriving from the premises isn’t enough.

Conditional Proof (CP)

The setup:

Basically, you use this method when the conclusion or a part of the conclusion you want is a conditional. This makes it so you assume the predicate in order to derive the consequent. Here’s an example:

Indirect Proof (IP)

The setup:

For this method, you use this primarily when the conclusion is a negated statement. You assume the un-negated form of the conclusion and attempt to find a contradiction so that the assumption is false, thus ending at the negated form. It also works the other way around where the conclusion isn’t negated so you make the assumption negated instead and then use the DN rule at the end. It’s also super useful when proving theorems where you have a limited plan of action. An example:

Theorems

Theorems are formulas that can be proven true without premises so the proofs for theorems have the additional challenge of not being able to build off of premises. Therefore, the above two methods are essential to be able to do proofs of theorems. Here’s an example:

RULES

1. All assumptions must be discharged(closed).
2. Lines between different assumptions must not cross.
3. Once discharged, steps within the subproof cannot be used anymore.

On to the next page for a few practice problems!

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