There is an interesting problem concerning component causes posed by Cartwright (1983) in her book How the Laws of Physics Lie, which is easily explained in terms of a simple example. Consider a cup sitting on the table. Why doesn’t it move? The explanation given by Newtonian mechanics is that the cup is experiencing two forces-the downward force of gravity and the upward ‘elastic’ force of the table-and these two forces exactly cancel to produce a zero resultant force. This zero (...) resultant force then produces a zero acceleration, which ‘explains’ why the cup doesn’t move. This rather simple, yet typical, example of theoretical explanation raises a surprisingly deep puzzle: What justification is there for believing in the existence of the component forces if they are redundant to the explanation? For it does not really matter what size the component forces are so long as their resultant is zero. (shrink)