The action outcome, x , may then be said to be dependent upon a combination of effective personal force and effective environmental force, thus

One is tempted to formulate the underlying relation between the two independent variables as an additive one, for if the effective environmental force is zero (which would mean that the combination of environmental factors neither hinders nor furthers the result at), then x will depend only on the effective personal force.

x = f (ff person, ff environment). | p. 82