The Argument from Knowability
The Sixth Way is taken from the idea of knowability. (1) Now it is certain that all things which are true are capable of being known. (2) But then we find that all truths are known. (3) For if it is true that there is an unknown truth, then a being is capable not only of knowing that truth but knowing that that truth is unknown; (4) and it is impossible that a being both know something and know that it is not known, for the latter is contrary to the former. (5) Therefore, there exists a being in the Universe that possesses intelligence by which all truths are known; and this we call God.
[see Fitch's paradox of knowability; credit due to Phil Atkins and others in the grad lounge (Luke and Ian?) for the original idea and suggestions on the wording]