Anselm is read for one argument, and the one argument is not the most consequential thing he did.

He was an Italian monk who became abbot at Bec in Normandy and then archbishop of Canterbury in 1093, a post he spent much of in exile after quarreling with two kings about who appoints bishops. The philosophy was written in the gaps.

Faith seeking understanding

This was the original title of the Proslogion and it is the program, so it is worth being exact about what it claims — because it is neither of the two things it gets mistaken for.

It is not believe, and stop asking. And it is not prove it first, then believe. The position is that faith comes first as a starting point, and that having started there, the understanding you then build is real understanding, arrived at by argument that has to work on its own terms.

His formulation: I do not seek to understand in order that I may believe, but I believe in order that I may understand. The arguments are not addressed to an unbeliever and they are not decorative either. They are a monk finding out what he is committed to, and they have to be valid.

Which explains a feature of the Proslogion that surprises people who meet the argument out of context: it is a prayer. The famous passage is addressed to God, in the second person, in the middle of a devotional work.

The argument

Stated once, because it belongs to him, and then handed on.

God is that than which nothing greater can be conceived. Even someone who denies God has that concept in mind — otherwise they could not deny anything in particular. But a thing that exists in reality is greater than the same thing existing only as a concept. So if that than which nothing greater can be conceived existed only in the mind, something greater could be conceived, which is a contradiction. Therefore it exists.

The standing treatment, with the later versions and the objections, is on the ontological argument. What belongs here is what is peculiar to Anselm's own statement: it is a priori, using no observation at all, which distinguishes it from every other argument for God in the period and is why it keeps coming back. Descartes, Leibniz, Gödel and Plantinga each produce a version, and each version is answering the same structural question — whether existence can follow from a concept.

Cur Deus Homo, which actually changed something

His other major work, and the one with real consequences. It asks why God became man, and it answers in a way that replaced the account the Church had been using.

The older picture was a ransom: humanity is held by the devil, and Christ's death is the price paid to get us back. Anselm found this grotesque — it makes the devil a party with rights — and argued instead in the vocabulary of his own society, feudal obligation.

Sin is a failure to render what is owed, and the gravity of an offense scales with the status of the one offended. An offense against infinite majesty therefore incurs a debt no finite creature can pay. But the debt is owed by humanity, so only a human may pay it; and it is of a size only God can pay. So the payer must be both, and the incarnation is not an arbitrary arrangement but the only available solution.

The satisfaction theory became the dominant Western account of the atonement, was reworked by the Reformers into a penal version, and is what most Western Christians now believe by default. It is the piece of Anselm that changed what was taught, and it gets a fraction of the attention the ontological argument gets.

The objection

Gaunilo, a monk of Marmoutier, replied while Anselm was alive, and the reply is printed in every edition because Anselm wanted it there.

His move was a parody: take the most perfect island conceivable. By the same reasoning it must exist, since an island that exists is greater than one that does not. Since it plainly does not, the reasoning is bad.

Anselm's response is that the argument only works for a being than which none greater can be conceived — an island is greater or lesser by degrees with no maximum, so there is no such thing as the most perfect island to reason about. Whether that answers Gaunilo or relocates the problem has been argued ever since, and Gaunilo's deeper point survives it: an argument that moves from a concept to an existence should make anybody nervous, and the burden is on the person who thinks the move is legitimate to say why it works here and nowhere else.

The blunter objection is historical. In nine hundred years the argument has persuaded, as far as anyone can tell, almost nobody who was not already persuaded — while remaining interesting enough that philosophers who reject it keep having to explain exactly where it fails, and keep disagreeing about that.