 Unfortunately, unrestricted invariance claims are rarely true. One example mentioned by Cartwright (1989, §5.2) in which they do indeed hold is the force law of classical physics, \(f := S_{i \in I}f_i = m \cdot a\), which equates the total force \(f\) with the sum of all component forces. In that case, an increase of a component force \(f_i\) will produce an increase of acceleration for all possible values of \(m\) and the remainder forces \(f_i (i\in I)\).  In Schurz (2002, §2, theorem 1) it is proved that \(X_i\) is connected with \(Y\) by an unrestricted comparative cp-law, where \(Y\) is a function of independent variables \(X_1 ,\ldots ,X_n\), iff the \(X_1 ,\ldots ,X_n\) are non-interacting causes of \(Y\) in a certain (technically defined) sense.