 To illustrate the independence of the two dimensions of type and source, consider the case of the unorthodox libertarian Hillel Steiner (1974–5, 1994). On the one hand, Steiner has a much broader view than Hayek of the possible sources of constraints on freedom: he does not limit the set of such sources to intentional human actions, but extends it to cover all kinds of human cause, whether or not any humans intend such causes and whether or not they can be held morally accountable for them, believing that any restriction of such non-natural sources can only be an arbitrary stipulation, usually arising from some more or less conscious ideological bias. On the other hand, Steiner has an even narrower view than Hayek about what type of obstacle counts as a constraint on freedom: for Steiner, an agent only counts as unfree to do something if it is physically impossible for her to do that thing. Any extension of the constraint variable to include other types of obstacle, such as the costs anticipated in coercive threats, would, in his view, necessarily involve a reference to the agent’s desires, and we have seen (in sec. 2) that for those liberals in the negative camp there is no necessary relation between an agent’s freedom and her desires. Consider the coercive threat ‘Your money or your life!’. This does not make it impossible for you to refuse to hand over your money, only much less desirable for you to do so. If you decide not to hand over the money, you will suffer the cost of being killed. That will count as a restriction of your freedom, because it will render physically impossible a great number of actions on your part. But it is not the issuing of the threat that creates this unfreedom, and you are not unfree until the sanction (described in the threat) is carried out. For this reason, Steiner excludes threats — and with them all other kinds of imposed costs — from the set of obstacles that count as freedom-restricting. This conception of freedom derives from Hobbes (Leviathan, chs. 14 and 21), and its defenders often call it the ‘pure’ negative conception (M. Taylor 1982; Steiner 1994; Carter and Kramer 2008) to distinguish it from those ‘impure’ negative conceptions that make at least minimal references to the agent’s beliefs, desires or values.