Aquinas’ Five Ways demonstrate God’s existence by arguing that he is the necessary condition of certain readily observable phenomena (ST Ia.2.3). Though each demonstration culminates in the claim that its subject is God, the rationale for these identifications does not emerge until subsequent investigations into their subject. Demonstration from a state of affairs to the existence of its cause is demonstration ‘‘that (quia)’’ whereas demonstration ‘‘propter quid (that on account of which)’’ explains a state of affairs through an insight into the essence of its cause. This latter form of demonstration is characteristic of science viewed as an organized body of knowledge, which is not to say that scientists adhere to the stipulations governing the construction of demonstrations as outlined in Aristotle’s Posterior Analytics, but rather that scientific explanation accounts for effects in terms of the natures of their causes. Aquinas admits that in this life we cannot acquire the knowledge needed to produce a demonstration propter quid with God as its subject; however, reasoning from creation viewed as God’s effect, we can provide a demonstration quia of his existence (ST Ia.2.2).
Aquinas’ reliance on states of affairs to demonstrate God’s existence breaks with the Anselmian claim that the proposition ‘‘God exists’’ is self-evident because its predicate is contained in a proper notion of the subject (ST Ia.2.1). Aquinas allows that a proper notion of the divine essence would render God’s existence self-evident, but denies the availability of such insight in this life; hence, God’s existence is self-evident ‘‘in itself (secundum se)’’ but not ‘‘to us (quoad nos)’’
Aquinas’ First Way observes that things are in motion or moved (the verb movetur allows either rendering). Following Aristotle, Aquinas recognizes three types of mobile change: in quantity, quality, or place (Sententia super Physicam V.3). From the general principle that anything in motion is moved by another, Aquinas concludes that there must be a first unmoved mover, for unless there is an exception to this general principle, we could ask for an explanation of every motion in terms of a previous mover ad infinitum, never arriving at a first mover whose activity is responsible for the existence of the last.
It is important to note that this discussion of first and last involves ontological rather than temporal priority. Aquinas’ example of a staff moved by a hand illustrates that the unfolding effect requires an agent’s ongoing activity; thus, the causal series is synchronic or ordered per se, in contrast with a diachronic series ordered per accidens (accidentally), where events are conceived as unfolding independently of their temporally prior antecedents, as one blow of a hammer follows another. Thus, Aquinas’ proof culminates with an unmoved mover continually actualizing the potentiality of all other movers (moving or moved), supporting Aquinas’ belief that God continually pours forth being into creation (SCG II.30.3).
Aquinas establishes that what is moved is moved by another through the description of change (or motion) as the reduction of potentiality to actuality. Every change is a generation (substantial or accidental) out of matter and potency, as when tinder catches fire. The reduction from potentiality to actuality requires an agent in actuality with respect to the form that is communicated. Tinder does not catch itself on fire, rather it is lit (and thus its potentiality actualized) by what is in actuality with respect to fire, say a match. Note that not every change involves an agent communicating a form it actually possesses. Alcohol, for instance, can inebriate but is not itself inebriated. Aquinas’ point is that within the cause there is an element in actuality with respect to the property communicated, inasmuch as when in actuality this element produces such and such a property in an appropriately disposed patient. Given an entity’s inability to communicate to itself a motion with respect to which it stands in potentiality (by means of that part of itself that is in potentiality with respect to the motion), the principle that every mover is moved stands, with the aforementioned caveat that an unmoved mover is required to account for there being now any motion. Moreover, as Aquinas subsequently notes, the first mover must be pure actuality; otherwise its activity would require the activity of another mover, and so on ad infinitum (ST Ia.3.1c).
Like the First Way, Aquinas’ Second, Third, and Fourth Ways demonstrate the existence of an entity that is pure actuality, the Second and Third as they conclude with the existence of an uncaused cause, and the Fourth, since it accounts for gradations of perfection in terms of an entity that is “maximally existent (maxime entia)’.’
Recalling Aquinas’ distinction between existence and essence, we may note that the first four Ways imply the identity of existence and essence in God, as any composition of existence and essence requires an accounting that involves potentiality within the composite (ST Ia.3.4c). Finally, as goodness, truth and being are, for Aquinas, convertible, he elsewhere argues that the first being, who is maximally good, possesses every perfection, including those Christians traditionally ascribe to the divine essence (ST Ia.4.2; SCGI.28.2).
World History
