Having dealt with the thought of Augustine of Hippo, we now turn to rational theology with an author who was able, with a single argument, to demonstrate God’s existence. This ontological argument, as it was later defined, retained its rational force until its definitive confutation by Immanuel Kant. In other words, its logical potential will only fail when faced with one of the most radical transformations of philosophy’s perspective of thought – the so-called Kantian revolution.
Anselm of Aosta, Bishop of Canterbury, was a thinker with a life full of events and political relations, both with the pontiffs and with the rulers of England. The role of bishop (1093), which he accepted only by obedience and so reluctantly, did not prevent Anselm from lavishing his great logical-rational capacity at the service of his faith, not only in his major works: the Monologion – ‘Soliloquy’ – and the Proslogion – ‘Colloquy’ -, but also his argument onTruth, Freedom of Will, The Fall of the Devil and others to which he added a remarkable corpus of wonderful ‘private letters’ and highly cultured ‘institutional letters’.
If id quo maius cogitari nequit exists as a thought, it is in intellectu, then it must necessarily also exist in reality, in re.
In the Monologion, the viewpoint is that of a non-believer, someone who does not know or accept the truths of Christian faith, and so as Amato Masnovo argued, God here is found not as a subject, but as a predicate. The viewpoint is the rationality of faith, i.e. the possibility of demonstrating the Truth of faith without necessarily assuming the truth of the Holy Scriptures; it could be defined as the practice of operational doubt: I doubt what I believe in order to allow rationality to freely demonstrate its truth without interference beyond rationality itself. As Sofia Vanni Rovighi once wrote, the term faith is to be understood as “an objective meaning of ‘fides quae creditur’, that is, of what is believed, and not in the subjective meaning of ‘fides qua creditur’” – the former is faith in the objective, rational, intelligible content of Christian doctrine; while the latter is subjective, personal faith that being the actual way in which an individual lives their religious experience. For these reasons, and others, Anselm’s works are an indispensable part of the journey of rational theology.
Proslogion begins with a proem, in which Anselm explicitly states the intent of what he calls his second booklet:
The ontological problem, therefore, is not creatio ex nihilo, as it was for Saint Augustine, but the actual existence of God as an entity – albeit quantitatively and qualitatively different from all others. The first chapter, in fact, is an exhortation to the mind: it must turn to God in order to contemplate him rationally, and thus capture an idea that can serve as a basis for rational thought. Already the second proposition of the second chapter (Quod vere sit deus) gives us the answer, which is essentially the ontological argument itself, but this will be better understood later. Let us therefore examine this passage in Anselm’s own words:
These few lines summarise the whole demonstration of the existence of God, which acts on the definition as a kind of implicit rational movement, or one that does not need to be manifested except in the face of an attempted confutation. The starting point is the definition of God’s nature, his essence one might say. One can try to go over it as follows: imagine you are an atheist, someone who does not believe in the existence of God but is equally fascinated by Christian doctrine. The first question to ask yourself would be: ‘what does a Christian think about when he or she thinks about God’? Anselm’s answer, and indeed there is no reason why this answer should not be considered the best, is: God is the greatest thing you can think of. That is to say, God is the summit of everything, he is the ceiling of all that exists, and it is not possible to think of something that is quantitatively greater and above all qualitatively better than God. At this point the atheist (whom Anselm calls ‘insipiens’, i.e. ‘foolish’) will be able to argue: ‘I accept this definition, but this thing of which we cannot think greater of, does not ontologically exist , i.e. it does not exist in the reality of things’. But Anselm fears nothing more: he already knows he has won the argument because implicitly, at the very moment in which the fool decides to reason on this definition, he or she has declared the existence of God. Anselm’s extraordinary logical mechanism surrounds the fool’s thought, like a trap, ready to be set it off.
Therefore, Anselm replies: ‘whether you accept or deny my definition, is not important, but you at least show that you understand it; if you understand it, then that definition exists in your intellect, that is, it exists as your thought’. In fact, even when a chef thinks of the dish he wants to prepare, it is evident that that dish does not yet exist in reality but exists in his thought as a sketch of the dish he is going to create. Therefore, things can exist in the intellect that do not exist in reality, or do not yet exist or no longer exist. This must be accepted by the fool, as it is difficult to deny. It is precisely with this small explication that the ontological argument comes to an end, reaching its unavoidable conclusion:
The logical movement is endowed with incredible power precisely because of its extreme simplicity: if id quo maius cogitari nequit exists as a thought, it is in intellectu, then it must necessarily also exist in reality, in re. Otherwise, if it existed only in the intellect, one could think of it as existing in the intellect and also in reality, and therefore one could think of something greater than that which cannot be thought of as greater. Simply, existing in the intellect and in reality, is more than existing only in the intellect; therefore it is an attribute that necessarily belongs to that which cannot be thought of as greater. Adopting a more contemporary language, one could say that the attribute of existence in re necessarily belongs to the maximum ontological entity, that is, to what is maximum with respect to everything else.
Anselm adds: “Assuredly, this [being] exists so truly [i.e., really] that it cannot even be thought not to exist [Quod utique sic vere est, ut nec cogitari possit non esse]”. 
A similarity should now ring in our minds with reminding us of the elenctic movement Aristotle spoke of in relation to the principle of non-contradiction. Remember how a feature of elenctic truth is the fact that it is undeniable, since it is reaffirmed even by its own denial. In this respect, even the Anselmian ontological argument partly echoes this structure, because it is precisely the denial that is expressed in the form: `what cannot be thought of as greater does not exist in reality’, which triggers the self-confirming mechanism of the statement `it necessarily exists in thought and reality’. There are, however, fundamental differences: firstly, the definition of God is the greatest thing you can think of. Secondly it needs to be recognised as existing in intellectu – i.e. its understanding must be assumed, and it would need to be clearly understood what ‘understanding’ means. In other words, if the èlenchos of the principle of non-contradiction applies to every proposition (since contradiction is impossible), here it seems that the mechanism works only for this precise definition of God, and that it operates a kind of ontological-rational bridge between the intellect and reality: rationality becomes the place where the existence of something that exists in reality is demonstrated, insofar as this thing is not perceptibly experienced.
The crucial point is the following: if one believes that one must deny the principle of non-contradiction, one will still be unable to contradict oneself, because whatever one says will be determinate, and therefore uncontradictory. On the other hand, after following the Anselmian ontological argument, if one decides to deny the existence of God, one can continue to live as if God does not exist, and this will be indeed possible. This is where the difference between the èlenchos and the Anselmian ontological argument becomes naïvely apparent.
However, in spite of this, the lucidity and rational rigour of Anselm, who struggles to demonstrate logically – and with such mastery – what he could have believed even sola fide, engages us to have a sense of gratitude towards him for having taken a decisive step forward on the path of philosophy and human thought.
 Anselm of Canterbury, Proslogion, in Complete philosophical and theological treatise of Anselm of Canterbury, translated by Jasper Hopkins and Herbert Richardson, The Arthur J. Banning Press, Minneapolis 2000, Preface, p. 88.
 Ivi, Chap. II, p. 93.
 Ivi, pp. 93-94.
 Ivi, Chap. III, p. 94.