  Duhem supports this statement using an analogy with the trajectory of a ball. We cannot guess its end point with an instant glance at the ball, but we can prolong its trajectory if we followed the ball from the moment it was struck. So we cannot guess the end point of physical theory, the natural classification, by looking at any particular theory. We must appeal to the trajectory of physical theory, to its history, to enable us to tell whether any particular theory is likely to contribute toward the ultimate natural classification. Duhem’s doctrine of natural classification provides the grounds for various commentators to argue that Duhem is not an instrumentalist, that his philosophy of science considered as a whole looks more like convergent or motivational realism (see Maiocchi in Ariew and Barker 1990 and others).