A number of ethical theorists in southern California (myself included) have recently convened a reading group to consider Derek Parfit's unpublished manuscript Climbing the Mountain. The book appears to defend a normative ethical theory that borrows elements of contractualism (in the Scanlonian reasonable rejectability vein), consequentialism, and Kantianism. I don't want to spill many beans, since the book is unpublished, but in his second chapter, Parfit presents some intruiguing ideas about Kant's Formula of Humanity.
That Formula famously states that we are always to treat humanity, whether in ourselves or in others, always as an end in itself and never as a mere means. The orthodox readings of the Formula tend to see Kant as proposing two distinct ways in which an action (or perhaps speaking in a more strictly Kantian sense, a maxim) can be wrong: It can treat another as a mere means, or it can fail to treat another as end in him/herself. One advantage of interpreting the Formula thus is that it establishes a parallel with Kant's distinction between perfect and imperfect duties, where violations of perfect duties are ways of treating a person as mere means (in the way that Kant thought that lying treated others as mere means) and violations of imperfect duties (e.g., when we fail in every case to give aid to persons in need) are ways of not treating persons as ends in themselves.
Now Parfit doesn't quite come out and say this directly, but the best reconstruction I can manage is that he believes he most plausible interpretation of Kant's Formula identifies only one overarching way in which our actions may violate it. (Parfit's emphasis is on actions, rather than on maxims or the attitudes that might be associated with actions.) That is, an action that does not treat a person as an end in itself is (or can be) wrong. Furthermore, an action that treats a person (a) as a means, and (b) fails to treat her as an end in itself, is (or can be) wrong. So on the view I attribute to Parfit, failing to treat a person as an end in him/herself is the fundamental concept behind Kant's Formula, and treating as a mere means is a species of not treating a person as end in itself. So to Parfit's eye, what distinguishes treating a person as a mere means from permissibly treating them as a means is that the former fails to treat the person as an end.
This intrigues me, simply because it strikes me as very standard to think that Kant meant to capture two species of wrong with the Formula, not one genus comprising two species. And it rejects the notion that there's something distinctive about treating a person as a mere means. But I'd be curious to know how others understand the Formula of Humanity, and in particular, how we ought understand the 'mere' in Kant's idea of treating as a mere means.