 While the inductive strategy of this work, directing us to high-level, convertible universals is, I hope, now clear, it is also important to note what is absent from this passage. Aristotle rarely in this work claims any of these correlations is necessary, or that others are impossible. He does not distinguish features that are part of the essence of a kind from others that are mere attributes; and he typically avoids any hint of causal explanation. All of this is to be expected, given the methodological guidelines for historia that we discussed earlier—historia is preliminary to causal investigation and prepares us for it. In the passage we are currently considering, for example, it is striking that he notes the convertible correlation between lung, windpipe and esophagus, on the one hand, and inhaling and exhaling of air, on the other, but does not claim that two of the organs are present for the sake of breathing. (In fact, the word for ‘breathing’ is never used!) Though he uses the term ‘nature’ regularly in HA, he does not, as he does so often in his causal investigations, claim that nature does nothing in vain, or that it always does what is best for each organism. Nor does he talk about the parts of an animal as its material nature, nor about its living capacities as aspects of its formal nature. Indeed there is virtually no mention of the matter-form distinction in HA. It is only when we turn to the various biological treatises devoted to causal investigation that this language associated with the search for definitions and demonstrations comes to the fore.