 This entry will focus on the contemporary discussion, but many of the ideas debated in the contemporary literature have been around for millennia. We now have the tools to make these important ideas much more precise. Relevant historical issues include ancient atomism (e.g., Leucippus and Democritus, see the separate entry on  ancient atomism),  Aristotle’s many discussions of priority (see, e.g., Peramatzis 2011 and the articles in Sirkel &amp; Tahko 2014), Aquinas’s discussion of the first cause (see the entry on the  cosmological argument),  and the  principle of sufficient reason,  as discussed by Spinoza and Leibniz, among others.