Following Donald Davidson, we suppose that cause produce their effects in accord with principles, at least wherever we can warrant causal claims. But we differ from Davidson in our view of what such principles are like. These need not be deterministic laws but, following J. S. Mill, ‘tendency principles’. Tendency principles are usually expressed as generics: they may only apply in appropriate contexts, they may not operate every time they apply (e.g. they may need triggering or act in a chancy way) and they don’t describe what actually happens since other tendencies may contribute as well. They can be found during research but can also be general knowledge. Knowing the principle and activity by which a cause produces its effect helps identify other features that can evidence causation.