It is well-known that Thomas Aquinas ceased writing his Summa Theologiae before completing it. When asked why, a long tradition recounts that he told his secretary, Reginald of Piperno: "After what I have seen today I can write no more: for all that I have written is but straw."
When some of my own students have used that quote as an excuse for not engaging in the demanding labor of theology, I've retorted: you can only say it when you've completed 7/8ths of the Summa.
In his fine new book on Aquinas, Thomas Aquinas: a Portrait, Denys Turner writes at greater length and with greater insight:
Theology matters only because – and when – there is more to life than theology, and when that "more" shows its presence within the theology that is done. So Thomas fails to finish, thereby exhibiting the presence of this "more" in the most dramatic way possible – by leaving space for it. His final sentence is not an empty and disappointing failure to finish. It is an apotheosis. By his silence Thomas does not stop teaching theology. He does not stop doing theology. On the contrary, by his silence he teaches something about doing theology that he could not have taught by any other means.