 Aquinas believes that natural reason can demonstratively prove God’s existence. The first step is to show that, for everything in the changeable world around us, there is a first cause, or prime mover, in virtue of which all other things have their existence, their motion, their qualities and direction. This is the result he gets in his much discussed “five ways” of proving God’s existence (ST 1a 2.3c). Aquinas denies that Anselm’s  ontological argument  has any force as a proof (ST 1a 2.1 ad 2; SCG I.10–11). Instead, the first three ways are  cosmological arguments,  based on the impossibility of an infinite series of causes. Commentators generally agree that what Aquinas means to exclude is an infinite series of simultaneous causes, a “vertical” series as opposed to an infinite “horizontal” series of causes going back in time, which he does not think can be proved to be impossible (§3). Each of the five brief arguments ends with a variation on the formula “this is what everyone calls God.” He puts his claim cautiously at this point, because to demonstrate the existence of a prime mover falls well short of demonstrating the existence of a being worthy of being called God. For all we have seen so far, for instance, this first cause might itself be a body. But Aquinas thinks natural reason—what we now call  natural theology—can  go well beyond the five ways. ST 1a goes on to establish that this first cause is not a body (3.1), nor composite in any way and hence entirely  simple  (3.7), perfect in every respect (4.2), the highest good (6.2), infinite (7.1),  omnipresent  (8.2), completely unchangeable (9.1), eternal (10.2), supremely one (11.4), and possessing  the most perfect knowledge  (14.1). These results establish the existence of something worthy of being called God. (On Aquinas’s natural theology in general see Kretzmann 1997; Wippel 2000 chs. 10–13; Stump 2003 pt. I. On the five ways see Kenny 1969; MacDonald 1991a; Martin 1997; Pawl 2012; Cohoe 2013.)