ABSTRACT

There is no fixed set of rules that every statement-logic system must have, though some (like MPP) seem indispensable. As always, the idea is to balance convenience against economy. Convenience depends on what kind of argument you will be wanting to analyse. Some systems contain the following rule, for example:

GIVEN P, we may conclude P or Q.

Show by truth-table that this is valid.

But although it can help in a formal proof, this rule is not much used in day-to-day argument. So I left it out. On the other hand, I have included rules that we do not strictly need, when they follow lines of thought we often use. For instance, we can prove this argument-form by MTT:

If P then Q Not Q __________ Not P.

But we could also use CONTRA + MPP:

1

If P then Q

PREMISE

2

Not Q

PREMISE

3

If not Q then not P

1 CONTRA

__________

4

Not P

2,3 MPP.