I would like to thank Dr. Osborne and Dr. Long for bringing the principle of sufficient reason up for discussion on Thomistica. I think it is a worthwhile discussion. My personal view is that some version of PSR is compatible with Thomism and can be "read back into" Aquinas's texts.
I am not going to defend that view now or get into the debate about whether the Leibnizian version of PSR is tenable or compatible with Thomism or how far Garrigou-Lagrange's concept of PSR is Leibnizian (or Spirian). I do not have the time for any of that at the moment but I hope to be able to have the time in the not too distant future.
For the nonce l would simply like to present and briefly comment on Norris Clarke's defense of a Thomistic version of PSR in The One and the Many (p. 181):
Some contemporary Thomists, like Gilson, insist it is against the spirit of Thomas to appeal to any general principle of sufficient reason. The reason they give is the danger of confusing it with the rationalist Principle of Sufficient Reason first explicitly introduced into modern philosophy by Leibniz, the great rationalist. But the Principle [as I understand it] is quite different from the Leibnizian rationalist one. The latter interprets the sufficient reason as some reason from which we can deduce by rational necessity the existence of the effect. It looks forward: given an adequate cause we can deduce the effect as flowing necessarily from it. It follows, of course, that no efficient cause can be free, and that God creates the world out of necessity, not freely, i.e., that to be rational God must create the best possible world. Our Thomistic interpretation is quite different. It does not try to deduce anything; it looks backward , i.e., given this effect, it needs such and such a cause to explain it. The cause must be adequate to produce it, be able to explain it once this is there. But in no way does this require that the cause has to produce it; in a word, our world needs an infinite Creator to explain it. But this in no way implies that such a Creator had to create it. It is not, like that of Leibniz, a deductive principle, deducing the effect from the cause, but as St. Thomas expresses it [sic!], like most other metaphysical explanations, it is a "reductive explanation," tracing a given effect back to its sufficient reason in an adequate cause. Given this key difference from any rationalist principle like Leibniz's. it seems to me that this general Principle of Sufficient Reason is a quite legitimate development of Thomism, with the advantage of summing up in one basic formula the principle of intelligibility of being that is implied in all of Thomas's specialized formulas of the Principle of Causality.
How does Clarke formulate PSR? Thus: "Every being has the sufficient reason for its existence (i.e., the adequate ground or basis in existence for its intelligibility) either in itself or in another" (p. 21).
I find nothing immediately to object to in what Clarke says. In fact, I am inclined to agree with his take on a Thomistic PSR although I do not know that I am prepared to endorse it definitively. I do also have to confess that I know very little about Leibniz's understanding of PSR, so for the present I will hold off on pronouncing on the validity of Clarke's interpretation of Leibniz.
I would be very happy for readers' comments on what Clarke says about PSR.