If in all cases where an effect occurs, there is a single prior factor X that is common to all those cases, then X is the cause of the effect.
Method of Concomitant Variation
We associate a quantitative change in the effect with a quantitative change in the resumed cause.
Method of Difference
Where one situation leads to an effect, and another which does not, and the only difference is the presence of a single factor in the first situation, you can infer this factor as the cause of the effect.
Method of Residues
If a variety of causes have been established to produce a variety of effects, and we have matched up all the causes, except one, with all the effects, except one, then the remaining effect can be attributed to the remaining cause.