Full Idea
It is much easier to trace effects back to causes with certainty than to predict effects from causes. If I have one contact with someone with a disease and I get it, we suppose I got it from him, but a doctor cannot predict a disease from one contact.
Gist of Idea
With diseases we easily trace a cause from an effect, but we cannot predict effects
Source
G.E.M. Anscombe (Causality and Determinism [1971], §1)
Book Reference
'Causation', ed/tr. Sosa,E. /Tooley,M. [OUP 1993], p.91
A Reaction
An interesting, and obviously correct, observation. Her point is that we get more certainty of causes from observing a singular effect than we get certainty of effects from regularities or laws.