I thought it might be interesting to open a designated Ockham questions thread for the holidays. I've started it with a few obvious ones. Feel free to criticize my responses, or ask your own questions.

What is Ockham's razor?

Ockham stated his principle of parsimony, or razor, a few different ways:

(1) “It is futile to do with more what can be done with fewer,” (Treatise on Quantity, ca. 1323-24)
(2) “When a proposition is made true for things, if two things suffice for its truth, it is superfluous to assume a third,” (Quodlibet, ca. 1323-25)
(3) “No plurality should be assumed unless it can be proven (a) by reason, or (b) by experience, or (c) by some infallible authority,” (Sentences commentary, ca. 1318-1319; Treatise on Quantity, ca. 1323-24; other places).

According to Professor Rondo Keele, experts generally agree that (3) is the closest to Ockham's actual razor[1]. Using these sources, he fills out (3) more precisely:

No extra-mental distinction among extra-mental things should be assumed unless the distinction can be proved (a) by arguments from premises that are either self-evident or else come from indubitable experience or, (b) by unquestionable experience of extra-mental things, or (c) by some infallible authority such as the Bible, the Saints, or certain Ecclesiastical pronouncements.

By removing the negation, the same three criteria provide Ockham's principle of plurality, or anti-razor:

An extra-mental distinction among extra-mental things should be assumed if the distinction can be proved (a) by arguments from premises that are either self-evident or else come from indubitable experience or, (b) by unquestionable experience of extra-mental things, or (c) by some infallible authority such as the Bible, the Saints, or certain Ecclesiastical pronouncements.

Whereas razors tell us to not have too much, anti-razors tell us to have enough. Together, Ockham's razor and anti-razor make up his rules of rational positing, and can be seen as his solution to the problem of ontological commitment.[2]

Does Ockham's razor tell us reality is maximally simple?

No. Call razors that say "reality is maximally simple" metaphysical razors. In his revised Sentences commentary, Ockham rejects metaphysical razors: 

There are many things that God does with more that He could do with fewer. Nor should any other explanation be sought. And it follows from the fact that He wills it that it is fitting and not futile to be done. (William Ockham, Volume 1, p. 159)[3]

Here is an argument for that rejection. Suppose it's true that “God made no more than He had to”. If the strong doctrine of creatio ex nihilo is correct and God created everything freely, then there is nothing God had to make. The strong doctrine of creatio ex nihilo and Divine Freedom are de fide teachings of the Catholic Church. Hence, there is nothing God had to make.

If God made no more than He had to and there is nothing God had to make, then God made nothing. Hence, from the initial assumption and the conclusion of the previous paragraph, God made nothing.

But since I exist, God did make something. Hence, God made nothing and God made something. Therefore, by reductio, it's not true that “God made no more than He had to”. So, Ockham didn't hold a metaphysical razor that says “Reality is maximally simple.”

Ockham held a methodological razor. He thought that we had reasons to keep our theories simple, even if reality isn't maximally simple.

Are you an Ockhamist?

No. I do, however, think we can learn from Ockham's work.


[1]Keele, Rondo. Ockham Explained.
[2]Quotes and translations for this section by Rondo Keele, for Ockham Explained. (He mentions Ockham's rules of rational positing in his Stanford article on Walter Chatton, as well.)
[3]Translated by Marilyn McCord Adams.