This chapter examines the fifth way of St Thomas Aquinas' Proofs of God's existence. The fifth way has an obvious similarity to the popular Argument from Design, but two differences should be noted at the outset. Each of Aquinas' arguments breaks down in its attempt to prove that every agent acts for the sake of an end; and without this a priori principle one may well be sceptical whether natural, non-living agents act for ends. Unlike the three arguments the Aristotelean argument from faults in nature seems a valid one: to regard something as a deficient specimen of a species, or to regard a natural process as having misfired, certainly is to look at nature with a teleological eye. The whole idea of teleological explanation has been for long suspect in philosophical and scientific circles. Sir Peter Medawar says firmly 'The idea that evolution has a main track or privileged axis is unsupported by scientific evidence'.