Reply to Gaunilo

Anselm's Liber Apologeticus contra Gaunilonem, more plainly titled in the manuscripts Quid ad haec respondeat editor ipsius libelli ("What the author of the book replies to this"), is his answer to the first serious objection raised against the argument of the Proslogion. Shortly after that work circulated from Bec, a monk named Gaunilo of the abbey of Marmoutier wrote a short response, Pro Insipiente ("On Behalf of the Fool"), taking up the cause of the one who says in his heart there is no God. Gaunilo's most famous move was the lost island: he could conceive of an island more excellent than any other, yet no one would conclude from that conception that the island exists. He also objected that one cannot really hold "that than which nothing greater can be thought" in the understanding at all, since it belongs to no known genus and is grasped only as a string of words. Anselm answered point by point, and then did something unusual: he instructed that Gaunilo's objection and his own reply be appended to every copy of the Proslogion, so that the argument would travel with its critique.

The reply's central insistence is that the argument works for one thing only. An island, however excellent, can always be thought to be surpassed, so the reasoning simply does not apply to it; Anselm says that if anyone finds anything else, in reality or in thought alone, to which the argument fits, he will give him the lost island never to be lost again. Only that than which nothing greater can be thought cannot be thought not to exist, because whatever can be thought not to exist, or thought to have a beginning or an end, or to exist here and not there, is thereby shown to be less than something that cannot be so thought. This shifts the weight from the bare claim that existence is greater than non-existence to the claim that necessary existence is intrinsic to the concept. Anselm also protests that Gaunilo had quietly replaced his formula with "greater than all things," which is a different and weaker notion, and he argues that the formula can be genuinely understood by anyone who can ascend in thought from lesser goods to greater, even without comprehending the nature so described. Throughout he addresses Gaunilo not as the fool but as a fellow Catholic, and appeals to his faith and conscience as the firmest witness that the formula is intelligible.

The exchange has mattered because it fixed the shape of the debate for nine centuries. Gaunilo's island and Anselm's reply are the template for every later contest over the ontological argument, from Aquinas's rejection and Descartes's revival to Kant's critique and the modal reconstructions of twentieth-century philosophers. The reply also makes visible what the Proslogion alone leaves implicit: that Anselm's real claim concerns a being whose non-existence is unthinkable, not merely one that happens to exist.

Who should read this: anyone working through the Proslogion, since Anselm himself refused to let the argument stand without it, and students of philosophy of religion who want the original terms of the dispute rather than textbook summaries. It is not an entry point to Anselm's spirituality and will frustrate readers who have not first read the Proslogion and Gaunilo's objection, on which every sentence depends.