This chapter explores particular causal scepticism about free will, which holds that the effect in a causal relation is essentially passive, and thus that the embedding of the agent in the causal nexus of the world eliminates the possibility of free will. When these causal relations are systematized in the concept of a mechanism, this scepticism takes the form of the objection that the elements of such a system are rigidly fixed in a way that is inconsistent with the reflective self-directedness of free will. Again this form of scepticism is attributed to a specific form of the principle of sufficient reason, one in which sufficient reasons cause what they explain, and this form of doubt about the reality of free will is traced through modern, 19th century, and contemporary sources.