Why? No one is entirely sure why but the last thing he wrote is this: All that I have hitherto written seems to me nothing but straw.
As a writer I understand this sentiment, but St. Thomas the Aquinas is no ordinary writer, scholar or thinker. As I said earlier, he was the Catholic Church's greatest theologian. Pope Benedict XV even once went so far as to declare: "The Church has declared Thomas's doctrine to be her own."
I suspect Pope Benedict wasn't including Thomas' final "straw" statement in this assessment — although it should. I say this because Thomas also once wrote that the ultimate in knowing God is "to know that we do not know Him."
This idea — which is a big one — requires a lot of unlearning. If more people could, however, embrace it, the world would be a better place because those who claim to "know God" are the ones who are really grasping at straws — and making a great mess of the world while doing it.