  Spinoza’s account of the affects (affectus) of the human mind is a response to one of the central problems for his naturalism. It is an attempt to show how the wide range of desires and emotions of the human mind can be produced by something which follows the order of nature. At the start of Part III (see also Chapter 2 of his Political Treatise), Spinoza notes that traditional accounts of the passions, with the exception of Descartes’s, have rested on the assumption—one wholly baseless in Spinoza’s view—that human beings are a separate “dominion” within the dominion of nature, with different kinds of constituents and governed by different sorts of laws.  Spinoza’s project continues what he finds to be Descartes’s important innovation: seeking “to explain human affects through their first causes.” So his account of the affects may be most profitably compared to Descartes’s in his Passions of the Soul. It may also be usefully compared to accounts in the writings of Hobbes (especially Leviathan VI), a contemporary who shared many of Spinoza’s philosophical commitments, or to some of the “traditional accounts” which Spinoza faults, such as Aquinas’s Summa Theologiae.  (Aquinas’s treatments of the passions appear mainly between Ia75 and 2a2ae189.)