DISJUNCTION ELIMINATION

Disjunction elimination is the formal name for the move 'either P or Q. Not P. Therefore Q.' Sherlock Holmes used it constantly: when you've eliminated the impossible, whatever remains, however improbable, must be the truth.

The practice trains you to USE disjunction elimination — but more importantly, to notice when you're tempted to use it on disjunctions that aren't actually exhaustive.

1. 1.Pick a situation where you're trying to figure out what's going on.
2. 2.List the candidate explanations as a disjunction: 'It's either A or B or C.'
3. 3.First check: is the list exhaustive? Are there explanations missing? (This is where most informal disjunction elimination fails — the 'A or B' was actually 'A or B or C or D' the whole time.)
4. 4.If the list is exhaustive, work through each: what would have to be the case for it to be true? What evidence rules it out?
5. 5.Rule out what you can rule out. Whatever remains is your tentative answer.
6. 6.Then test the remainder: even if you've eliminated the others, does this one fit the evidence well?