The human enhancement debate has over the last few decades been concerned with ethical issues in methods for improving the physical, cognitive, or emotive states of individual people, and of the human species as a whole. Arguments in favour of enhancement defend it as a paradigm of rationality, presenting it as a clear-eyed, logical defence of what we stand to gain from transcending the typical limits of our species. If these arguments are correct, it appears that adults should in principle (...) be able to make rational and informed decisions about enhancing themselves. In this paper, however, we suggest that a rational and informed choice to enhance oneself may in some cases be impossible. Drawing on L. A. Paul’s work on ‘transformative experience’, we argue that some enhancements—such as certain moral or cognitive modifications—may give rise to unbridgeable epistemic gaps in key domains. Importantly, such gaps could prove to be not merely contingently unbridgeable due to a lack of information at a given moment, but radically unbridgeable, making someone in a non-enhanced state inherently unable to conceive of what it would be like to be enhanced in a particular way. Where this experience is key to understanding what values are being pursued by the enhancement itself, it may prove impossible for a person to be sufficiently informed, and to make a rational decision about whether or not to enhance herself. This poses a challenge for human enhancement proponents in general, and for transhumanists in particular. (shrink)