Your question is very good and not silly at all. And I think that people often make mistakes in it. There are a few ways to logically support the claim of one factor causing another.
1) Knowledge of the subject. You may know enough about the subject to know causality without relying on statistical evidence.
2) A designed experiment. You may be able to control and manipulate one factor in a well-designed experiment and show statistically that the controlled factor causes the result in the other factor.
3) There appear to be other statistical ways to support causality that I am not familiar with. See
this.