 Putting aside these issues about possibility for the present, the most natural way of defining the notion of causal effect in the framework associated with  (M1)–(M4)  is in terms of the difference made to the value of \(Y\) by a change or difference in the value of \(X\). (This is also effectively the definition of causal effect adopted in Rubin 1974. Focusing on differences in this way allows us to isolate the contribution made to \(Y\) by \(X\) alone from the contribution made to \(Y\) by its other causes. Moreover, since in the non-linear case, the change in the value of \(Y\) caused by a given change in the value of \(X\) will depend on the values of the other causes of \(Y\), it seems to follow that the notion of causal effect must be relativized to a background context \(B_i\) which incorporates information about these other values. In deterministic contexts, we might thus define the causal effect on \(Y\) of a change in the value of \(X\) from \(X=x\) to \(X=x'\) in circumstances \(B_i\) as: