...I think, therefore I am (COGITO ERGO SUM)...
   René Descartes, Discourse on the Method, Part IV, trans. John Veitch. Project Gutenberg #59

Thoughts without content are void; intuitions without conceptions, blind.
   Immanuel Kant, Critique of Pure Reason, trans. J. M. D. Meiklejohn. Project Gutenberg #4280

Whatsoever is, is in God, and without God nothing can be, or be conceived.
   Baruch Spinoza, Ethics, Part I, Prop. XV, trans. R. H. M. Elwes. Project Gutenberg #3800