 It would be universally agreed by scholars that substantial forms are real in the sense that they play an irreducible and ineliminable explanatory role in the behaviour of the things in which they are the form. There are two interpretations of what Aristotle meant by this, one of which seems compatible with modern science and the other not. First the compatibilist one. Empiricist philosophers of science used to believe that the concepts in higher order sciences could be reduced, either by means of reductive definition or by ‘bridging laws’, to those in the more basic sciences (see, for example, Nagel 1961). By such means, everything could be understood in terms of the most basic science—presumed to be physics. Concepts in the other sciences are thus only a kind of shorthand for laborious descriptions in the language of physics. Few, if any, philosophers of science believe this now. They agree that, even if the world is ‘closed under physics’—every event has a complete set of physical causes—the concepts of the other sciences are irreducible and do autonomous explanatory jobs. Even if dogs are wholly physical objects and even if all the atoms in dogs follow very precise laws of physics, nevertheless when doing the biology of dogs one will need concepts not to be found in a physics textbook. This is not for mere shorthand convenience, but because of the kinds of things in which, for example, the biologist or veterinary scientist is interested. There are interpreters of Aristotle who think that this kind of irreducibility is all that Aristotle means—or needs to mean—by postulating an explanatory role for substantial form. His theory is at least neutral on the question of whether there is a closed system at the level of basic physics (Nussbaum 1978, 1984).