 With this sketch of practical reasoning we can come to closer grips with what sorts of things could not be otherwise. Let us begin with health and the claim that the physician has an account of health. It is health that is the cause of the steps in the practical reasoning because it explains what is to be done. In what way does health admit of being otherwise? One possibility is that what constitutes health is not invariable; thus the account of health would not be invariable. While it seems unlikely that health is variable in this sense, what obviously could be otherwise is whether health exists in this particular case or not. If health does not exist in this case, it is up to medicine to restore it. In this sense, as well, all of the steps prescribed by the practical reasoning could be otherwise. In the case of this patient, homogeneity may or may not exist; heat may or may not exist. However, there is another sense in which the steps in the syllogism might admit of being otherwise. Each step alleges a productive relation — e.g., heat produces homogeneity. Now, there is a sense in which these relations could be otherwise. While heat produces homogeneity, in a particular case heat causing homogeneity may or may not exist because there may or may not be heat. To say of all these conditions that they could be otherwise, then, implies that they could or could not exist. Whether these conditions exist or not depends on an agent bringing them into existence. So the field of craft is those conditions that can be brought into existence by an agent.