Is this particle's acceleration due more to gravity or to electricity? Classical physics regards this question as well-conceived. We may answer by examining physical details of the system before us and using our knowledge of pertinent laws. Newton's law of gravity tells us how mass produces a gravitational force; Coulomb's law of electricity shows how charge generates an electrical force. Each of these source laws can be connected with the consequence law “F = ma” to determine which force induces the greater component acceleration.

In this Newtonian case, two questions seem interchangeable: What contribution did gravity (or electricity) make to the particle's acceleration? What difference did gravity (or electricity) make in the particle's acceleration? These questions are simultaneously addressed by investigating two counterfactual questions: How much acceleration would there have been, if the gravitational force had acted, but the electrical force had been absent? How much acceleration would there have been, if the electrical force had acted, but the gravitational force had been absent? Classical particles obey John Stuart Mill's (1859) principle of the composition of causes. The result of the two forces is just the sum of what each would have achieved, had it acted alone. We see what each contributed by seeing what difference each made in the magnitude of the effect.