The first type of arguments, those from hierarchy, took their departure from a metaphysical framework with roots in Neoplatonism. A point on which most medieval philosophers would distance themselves from the important Neoplatonic heritage, however, was the question of the existence of the Supreme Being. According to Plotinus, the One is so high on the scale of beings that the One is beyond being, and therefore, properly speaking, the One is not at all. While retaining the Platonic notion of a hierarchy of being, the medievals would hold that God occupies the top level of the order of the things there are. Proving the existence of God within this framework is essentially a matter of establishing that there is indeed a single highest entity in the hierarchy, and that this entity is identical to the monotheistic God.
Augustine puts forward an argument for God’s existence in De libero arbitrio, book 2. The argument focuses on the supposed divine properties of eternity and immutability. If it can be shown that there exists some one thing eternal and immutable which is also superior to human reason, then God exists. Such a thing does indeed exist, Augustine argues, and this thing is truth. Some things are eternally and immutably true, namely, necessary truths such as those of mathematics. What makes the true things true is truth itself, and since the truths made true by truth are eternally and unchangeably true, truth itself must be eternal and unchangeable. Furthermore, truth is superior to human reason. According to Augustine, truth imposes a norm on human reason by which human reason must abide, and so, metaphorically speaking, truth is the judge of reason; therefore it is better, and so higher on the hierarchy of being. There is an interesting twist to the last stage of the argument. Where one would perhaps expect Augustine simply to identify God with truth itself, Augustine hedges the question and argues that either God is truth, or if there is something higher still, then this higher thing is God.
The first argument for the existence of God in the Middle Ages proper can be found in Candidus Wizo, a student of Alcuin. In a short work titled Dicta candidi de imagine dei, Candidus argues, following Augustine, that all things can be divided into what exists, what lives, and what understands. The three classes of things constitute an order in terms of both power and goodness. An animal, which both lives and exists, is more powerful and also better than a stone, which merely exists. A human being, who has understanding, is better and more powerful than both. When the human being realizes that his own power has limits, he must admit there is a being who has the power to rule over the very existence of what lives and understands, and this being, Candidus adds, is God.
In the Monologion 1-3 Anselm of Canterbury argues from the existence of good things to the existence of a being Supreme with respect to goodness, greatness, and being. All good things are good through, or by virtue of, some one thing. There is one thing, which makes all good things good (reification of property). Moreover, this thing is itself good, indeed good to the highest degree (selfpredication). Since it is the best thing it is also the greatest thing, in the sense that no other thing is equal or surpasses it in goodness. Further, all things that exist exist through something, and it cannot be the case that they exist through many separate things and there cannot be any existential cross-dependencies. Thus, there is some one thing on which all things, including itself, depend for their existence. Dependent existence is according to Anselm a lesser form of existence than self-dependent existence, so the thing on which all other things depend, is itself existent to the highest degree.
In the Proslogion, Anselm puts forward his most famous and influential argument - perhaps one of the most discussed pieces of reasoning of all time. If one understands God to be that than which a greater cannot be thought, one must, according to Anselm, also accept that God is. To think that that than which a greater cannot be thought is only in the understanding (in intellectu) is to think incoherently, since it is greater for it to be both in reality (in re) and in thought.
Anselm’s first critic was the contemporary Gaunilo of Marmoutiers, whose response On Behalf of the Fool was circulated as an appendix to the Proslogion, together with replies to Gaunilo by Anselm. Gaunilo raises several questions, focusing particularly on what follows from claiming that something is understood, or, as Anselm puts it, that something is in the understanding. The gist of his critique is that the existence of a thing can never be adduced from thought or concept alone. He illustrates his point by arguing that if Anselm were right, one should be able to argue from the mere thought of an island most excellent of all to the existence of that same island.
The rediscovery and dissipation of the full canon of Aristotle’s organon and his Metaphysics in the mid-twelfth century onward was accompanied by an increased focus on problems of epistemology, against the background of which Anselm’s argument would be viewed in new ways. Two major epistemological approaches dominated: Augustinian theories of Divine illumination, and Aristotelian theories of abstraction. According to the former, human concept formation depends at least partly on direct supernatural assistance; according to the latter, concepts (intelligible species) are the result of a cognitive process of abstraction, which depends on data supplied by the senses. On neither theory is there suggested a way for human beings to gain a concept of God by natural means alone. Most often, scholastics following either approach would hold that it is possible for humans to know that God is, but not what He is, that is, his quiddity or essence. It seems therefore Anselm’s formula ‘‘that than which a greater cannot be thought’’ is either not a proper concept of God, or, if it is, must have been supernaturally implanted. Defenders of Anselm’s argument such as the authors of the Summa fratris Alexandri Halensis and Bon-aventure would indeed claim that Anselm’s argument is valid, but they also argue that our knowledge of God’s existence is a naturally impressed disposition, or that the claim that God exists is self-evident, and thus the point of putting forward an Anselmian argument starts to look questionable. Nevertheless, Bonaventure took serious interest in Anselm’s argument, and put forward several versions of it, formulated against the background of the doctrine of the transcendentals. For instance he argues, in his Commentary on the Sentences, that truth is a property of God, and therefore God’s being (or God’s ‘‘to be’’: his esse) is so true that God cannot be thought not to be (bk. 1 dist. 8, art. 1, q. 1). He also put forward a rejoinder to Gaunilo’s beautiful-island argument, arguing that the notion of an island is the notion of an inherently imperfect, inherently limited, categorical being while God must be described through the transcendentals, which do not imply imperfection. The concept of a perfect island is therefore incoherent and so cannot rationally be thought of as a thing than which no greater thing can be thought. (De mysterio trinitatis q. 1, art. 1, ad 6).
Thomas Aquinas suggests in the Summa theologiae that there are five ways to prove God’s existence, none of which follow Anselm’s Proslogion argument. Aquinas’ fourth way is, however, in the tradition of Anselm’s strategy in the Monologion, starting from the observation that different things exhibit goodness, truth and being to different degrees, to the conclusion that there must be some one thing, itself the most accomplished with respect to goodness, truth, and being, which is the cause of all other, and this is God. While sympathetic to proofs based on the notion of a hierarchy, Aquinas appears to explicitly reject the Proslogion argument in both SCG and ST, although the exact target is perhaps not so much Anselm as Anselm’s thirteenth century advocates. According to Aquinas, God’s existence is not self-evident (per se notum) to human beings, neither in the form of an implanted piece of knowledge nor as a conclusion drawn from self-evident premises. The latter would require knowledge about God’s essence, to which man lacks access. Instead God’s existence must be proved by appeal to God’s effects, that is, observable facts of the sensible realm of creation.
Scotus, on the other hand, considered a ‘‘touched-up’’ version of Anselm’s argument a valid proof for the existence of a first (i. e., highest) being in the order of eminence. According to Scotus, that than which nothing greater can be thought ‘‘without contradiction’’ can be readily proved to exist (e. g., De primo principio 4.65). However, proving the existence of such a best being is merely a part of Scotus’ proof for the existence of God as an infinite being (see below).
World History
