Chapter Eight, Tutorial Three
Strategy for PD is very much like that for SD and SD+.
One first provides a goal, then second considers
"obvious" ways to reasonably proceed toward that goal.
Third, when steps are not obvious, one considers the I-rules for the main connective of the goal at hand and E-rules for the main connective of any accessible sentences.
If all else fails, the "desperation" step, fourth, is to use ~E: assume the negation of your current goal.
That's basically all that's needed for PD too. But lets look back to a very easy illustration (from the last quiz).
How should we think about this problem?