Since the millennium, philosophical work on human enhancement has burgeoned. Significant book-length treatments have been published by, among others, Nick Bostrom, Francis Fukuyama, Jürgen Habermas, Michael Sandel and Julian Savulescu. Clearly, the idea that technology affords or will afford monumental changes in our physical, cognitive and even moral constitution has excited deep and widespread interest. Whether this interest is accompanied by profound scepticism, mere ‘boosterism’, or something in between, the prospect of manifold enhancements in our bodies and their capacities is hard to ignore. In this paper, I will explore what I take to be the four main arguments in the literature against such enhancements: namely, the argument from autonomy, from dignity, from inequality and from mastery. While each raises legitimate concerns, I will conclude that none of them – taken either singly or jointly – is sufficient to render the project of human enhancement impermissible. At most, they point to the need for prudence and careful institutional oversight. The only argument which succeeds against that project is not moral, but rather formal in kind: that is, that the project of human enhancement is fundamentally incoherent. I will argue that in order to specify an enhancement of x, one needs to specify (and understand) the nature of x. But it is precisely a specification and understanding of human nature which the project of human enhancement lacks – and worse, which it tends to repudiate.