  It is important to note that the fourth premise plays a crucial role  not only in Leibniz’s proof, but also in the larger picture of what  he takes his proof to show. Early in his career, Leibniz took the  principle of the equality of cause and effect to be a necessary   truth.[18]  Later, however, he came to hold that it is only hypothetically or  morally necessary – a contingent feature that must nonetheless  be found in the best of all possible worlds (see, for example, G IV  281/L272). From the status of the principle of equality of cause and  effect, Leibniz in turn drew two important conclusions, one  metaphysical, one methodological. With respect to metaphysics, he  took the contingent status of the principle of equality of cause and  effect to be evidence that the laws of motion and force are  themselves contingent, and thus he argued that the elegance of the  laws of nature provides evidence of God’s benevolent design of the  universe. With respect to methodology, he took the derivation of the  conservation of vis viva as confirmation of the utility  – indeed the practical necessity – of considerations of  divine teleology in making scientific discoveries even in the domain  of physics. 