"Human reason has the peculiar fate in one species of its cognitions that it is burdened with questions which it cannot dismiss, since they are given to it as problems by the nature of reason itself, but which it also cannot answer, since they transcend every capacity of human reason" (original Preface to The Critique of Pure Reason).

"Although there are many things that I think with the clearest conviction and utmost satisfaction but shall never have the courage to say, I shall never say anything that I do not think" (1766 letter to Moses Mendelssohn). 

Critique of Pure Reason