The three stages of using evidence
Stage 1: Use only strong, direct evidence.
Proves propositions conclusively.
Stage 2: Allow strong, indirect evidence.
Cannot prove but can show that proposition is the most reasonable answer.
Stage 3: Allow weak (uncertain) evidence.
Can do nothing because all weak evidence may be wrong.
Sequence
Begin with Stage 1.
Move to Stage 2 if necessary.
Do not move to Stage 3.