This paper examines the neo-Aristotelian account of species natures as ‘life-forms’, which we owe to Philippa Foot, Michael Thompson and their defenders. I begin by developing two problems for their view: a problem of underdetermination and a problem generated by psychological work on ‘folk essentialism’. I move on to consider their important transcendental argument, which suggests that claims about life-forms are presupposed by all efforts to describe the organic world. In response, I sketch a neo-Kantian projectivist position, which agrees that (...) life-forms are presupposed in these contexts, while denying that such life-forms are real. This position makes a better sense of the phenomena cited in support of the neo-Aristotelian view, while avoiding the problems raised for that view in the first half of this paper. (shrink)