and what is "accidental", or not by nature, is important in Aristotle's understanding of nature. As pointed out by Sedley, "Aristotle is happy to say (Physics II 8, 199a33-b4) without the slightest fear of blasphemy, crafts make occasional mistakes; therefore, by analogy, so can nature." According to Aristotle the changes which happen by nature are caused by their "formal causes", and for example in the case of a bird's wings there is also a final cause which is the purpose of flying. He explicitly compared this to human technology: The question of how to understand Aristotle's conception of nature having a purpose and direction something like human activity is controversial in the details. Martha Nussbaum for example has argued that in his biology this approach was practical and meant to show nature only being analogous to human art, explanations of an organ being greatly informed by knowledge of its essential function. Nevertheless, Nussbaum's position is not universally accepted. In any case, Aristotle was not understood this way by his followers in the Middle Ages, who saw him as consistent with monotheistic religion and a teleological understanding of all nature. Consistent with the medieval interpretation, in his Metaphysics and other works Aristotle clearly argued a case for there being one highest god or "prime mover" which was the ultimate cause, though specifically not the material cause, of the eternal forms or natures which cause the natural order, including all living things. He clearly refers to this entity having an intellect that humans somehow share in, which helps humans see the true natures or forms of things without relying purely on sense perception of physical things, including living species. This understanding of nature, and Aristotle's arguments against materialist understandings of nature, were very influential in the Middle Ages in Europe. The idea of fixed species remained dominant in biology until Darwin, and a focus upon biology is still common today in teleological criticisms of modern science. Roman era It was the Stoics who "developed the battery of creationist arguments broadly known under the label 'The Argument from Design'". Cicero (c. 106 – c. 43 BC) reported the teleological argument of the Stoics in De Natura Deorum (On the Nature of the Gods) Book II, which includes an early version of the watchmaker analogy, which was later developed by William Paley. He has one of the characters in the dialogue say: Another very