This “potentially causal variable” is called, traditionally, the “independent variable.” With the current example, that would be the coffee.

This paper makes explicit and takes issue with the bizarre view, which is unfortunately prevalent among social scientists, that causal relations are features of models only. There are some good ...

Logging dependent and independent variables has the added attraction that slope coefficients may be interpreted as elasticities. In the event that the dependent variable assumes nonpositive values, ...

For example, a significant correlation between the independent and dependent variable does not necessarily indicate a cause and effect relationship.