Chapter 2 analyses Hobbes’s ‘doctrine of necessity’, which was famously so termed by Hobbes himself in his debate with Bishop Bramhall. It argues that by means of this doctrine’s basis in the natural sciences Hobbes transformed natural law from an idealist to a pragmatic enterprise, from one based on natural rights to one characterized by the pragmatic employment of principles of necessity. The chapter discusses Avicenna’s metaphysics of existence, through which he described ‘nature’, which gives precedent to what exists in the material world and depends on the necessitarian principle that ‘whatever exists is necessitated by another’. Similarities between Avicenna’s metaphysics and Hobbes’s are analysed, and in respect of the former the capacity of the doctrine of necessity to sustain a metaphysical framework for philosophy and politics is identified.