We argue that Reasons Fundamentalism, as defended by T.M. Scanlon, faces two important difficulties—one ontological, the other epistemological—namely the ontological proliferation problem and the reliability challenge. We suggest that formal constructivism can avoid these difficulties, and that Scanlon would do well to adopt it. We also show that Scanlon’s three main objections to this view depend either on a misunderstanding of what formal constructivism is or on a question-begging argument in favour of realism.