“Religion and philosophy fruitfully cooperated in the Middle Ages. Philosophy, as the handmaiden of theology, made possible a rational understanding of faith.”
Medieval philosophers:
Augustine, Boethius, Anselm, Bernard de Clairvaux, Peter Abelard, Avicenna, Averroës, Moses Maimonides, Robert Grosseteste, Roger Bacon, William of Auvergne, Bonaventure, Albertus Magnus, Thomas Aquinas, John Duns Scotus, William of Ockham, Meister Eckehart, Nicholas of Cusa
Notable:
St Anselm of Canterbury (11th Century), because of his Ontological Argument
William of Ockham (14th Century), because of Ockham’s Razor
St Anselm of Canterbury, 11th Century
Anselm originated the notorious Ontological Argument, an alleged a priori proof of the existence of God, adopted later by the Rationalists (Descartes, Spinoza, and Leibniz).
Anselm’s Argument
God exists in the understanding.
A being greater than God cannot be conceived since God, by definition, is a being than which no greater can be conceived.
If God exists in the understanding but not in reality, a being greater than God can be conceived, namely, a being just like God except He exists in reality.
Therefore, God exists in reality.
Objections
Refutation by Logical Analogy is a method of refuting an argument, by setting forth a patently invalid argument of the same logical form. Guanilo, a contemporary of Anselm, put forth this analogous, absurd argument:
The perfect island exists in the understanding.
If the perfect island exists in the understanding but not in reality, an island greater than the perfect island can be conceived, namely, an island just like the perfect island except that it exists in reality.
An island greater than the perfect island cannot be conceived, since the perfect island, by definition, is an island than which no greater can be conceived.
Therefore, the perfect island exists in reality.
Later, Kant gave the most insightful refutation of Anselm’s argument, that “existence is not a predicate.”
William of Ockham, 14th Century
A conspiracy theory explains events by invoking a secret plot by powerful conspirators. The default argument against such theories is that there is always a simpler theory: that things are just as they seem, e.g. that astronauts really walked on the Moon, that Obama was really born in the US, that the Holocaust really happened. The default argument uses the Principle of Simplicity, that the simpler of competing theories is to be preferred, other things being equal.
William of Ockham put forth a version of the principle, Ockham’s Razor:
pluralitas non est ponenda sine necessitate
meaning plurality should not be posited without necessity.
Or, as it’s usually put:
Entities should not be multiplied beyond necessity
A Philosopher's View 