To put it at its weakest, we may say that there are discussions of biological phenomena in which the teleological account appears to recede and an account in material terms seems to be all that is thought relevant. But it seems possible to go further. Whereas—at the risk of simplifying a highly complex topic—interpretations of Aristotle would tend to stress the co-existence and co-operation of the different causes, or at least of the material and the final and/or formal, in causal accounts in the natural world, certain Galenic passages seem to suggest (while making specific reference to Aristotle) that different types of causal account may be applicable at different levels within the natural or biological world. Considering, in the context of this discussion of bodily mixture, what constitutes perfection in a human being, Galen states: