Sed nescio quo modo nihil tam absurde dici potest quod non dicatur ab aliquo philosphorum. (There is nothing so absurd but some philosopher has said it.)
Cogito cogito ergo cogito sum -- "I think that I think, therefore I think that I am;" as close an approach to certainty as any philosopher has yet made.
Philosophers, for example, often fail to recognize that their remarks about the universe apply also to themselves and their remarks. If the universe is meaningless, so is the statement that it is so.