I just received some feedback on a paper in which I present some results of the character project I have been working on in the past few months, and one of the worries expressed by the reader was that my account seemed too extreme in its bottom-up, materialistic account of character – so much so that it seems to violate Aristotle’s usual hylomorphism, according to which soul-capacities have both a formal and a material component and where both these components play a (perhaps more equal) causal role. This is a fair concern, so let me try to explain what kind of thing I believe natural character is, and what role material and formal factors play in its explanation.
As I indicated in an earlier post, Aristotle treats character in his biological works as one of the four differentiae that make animals the specific kinds of living beings they are (see, e.g., HA I 1, 487a11-12) and that therefore can be used in the causal explanation of some of their other features. The fact that natural character is a differentia does not mean, however, that it is necessarily an essential characteristic of a whole animal kind, i.e., that it is intrinsically or primarily connected to the form or the formal nature of that animal (as, for instance, ‘being a flyer’ is for birds – having this capacity is part of the bird’s substantial being). For, as Balme (1987, 19) has pointed out, some of the differentiae used in the History of Animals are ‘variable accidents’ or sub-specific characteristics, which can be used in the explanation of local differences within animal kinds and which are often primarily connected to the material nature of animals (think, for instance, of the differences in the coloration and pitch of voice among ducks, which are of the kind of affections Aristotle discusses in Generation of Animals V and which are explained almost entirely in terms of material-efficient causation). In addition, there are soul-functions that are formally determined at a very general level, but the species-specific mode of which – which then counts as a differentia of that species – is determined bottom-up, by the material nature each species turns out to have.
Take the example of ‘reproduction’. While the general capacity to reproduce is itself a formal property of every living being (each living being must by definition possess a nutritive soul, which includes the capacity to reproduce; all birds are thus by definition ‘reproducers’ of some kind), the specific mode of reproduction by which an animal species produces its offspring is something Aristotle explains by reference to their species-specific material natures. That is, in GA II 1, 732a25-733b23, Aristotle identifies the differences in the material natures of different kinds of animals as the cause for the differences in the perfection of their modes of reproduction and hence for the kind of reproducer they end up being (see especially GA II 1, 732b27-29). Thus, the more heat the animal has and the less earth it has in its material constitution, the more perfect the mode of reproduction will be (i.e., the more complete the offspring will be by the time it is cast out of the mother, both in terms of sexual differentiation and in terms of the formation of all its necessary parts). In other words, whether an animal species turns out to be an ‘egg-laying reproducer’ rather than a ‘live-bearing reproducer’ is determined bottom-up, by its material nature, and not top-down by its soul (as far as I can tell, Aristotle nowhere suggests that some animals have to be colder and earthier because this is conditionally necessary for the sake of producing eggs; instead, laying eggs is depicted as a consequence of their material nature), although, of course, the fact that an animal is an egg-layer and not a live-bearer is part of its nature, just as being a sexual reproducer is part of its nature.
I believe that Aristotle’s account of ‘natural character’ is analogous to his understanding of reproduction. Just as in the case of reproduction, it seems plausible that Aristotle holds that to possess character is a necessary formal feature of every animal – i.e., every animal will need to have a certain way of responding to its environment, which will correlate with its way(s) of life and activities, and which will be a part of its soul (cf. PA IV 11, 692a22-23: τὸ τῆς ψυχῆς ἦθός), just as it will need to have a certain mode (sexual or not) of reproducing itself. However, Aristotle’s references throughout the biological works to the level of perfection of material natures and to the material properties of the blood of an animal as causally basic facts with regard to character indicates that the species-specific kind of character an animal has is not determined by its form but by its material nature, which is itself not conditionally necessitated but is rather an independent basic fact about that kind of animal. (For instance, it is not conditionally necessary for cuckoos to have a cold material nature, in order that they may realize their cowardly character. Rather, as Aristotle explains in GA III 1, 750a12, the species-specific character the cuckoo has is a consequence of its cold material nature: the cuckoo is cowardly because of its cold nature, it is not cold for the sake of being cowardly.)
In sum, both formal and material factors play a causal role in Aristotle’s account of natural character, but for our understanding of why animal species – including human beings – have the specific character traits they have, the differences in the material natures of animals are most important.
 Birds are thus sexual reproducers due to the kind of formal nature they possess, but they are egg-layers due to the kind of material nature they possess, and both are characteristic features of birds. See Devin Henry’s “Aristotle’s Pluralistic Realism”, forthcoming in the Monist, on this example and on how it is the possession of a certain kind of material nature that unifies egg-layers as a natural kind in Aristotle’s natural science. For material features being part of the nature of a species, without being a formal property of them, see cf. GA V 6, 785b16-786a2 and especially 785b30-31: ‘for it belongs to the whole species not to have one color in their nature (διὰ τὸ ὅλῳ τῷ γένει ὑπάρχειν ἐν τῇ φύσει τὸ μὴ μίαν ἔχειν χρόαν).
 See Lennox (2010), 329-355. Note that Aristotle treats character as a differentia that is explanatorily basic and with reference to which other differentiae are teleologically organized; see, for instance, HA VIII 1, 588a17-18, where Aristotle explains that ‘the activities and ways of life differ in accordance with the characters and foods.’