The pun can be any number of reference to his theories, but the one I remember is "Can you understand the ultimate nature of reality? I can't (or I Kant)."
Here's a little background from Wikipedia:
Immanuel Kant (22 April 1724 – 12 February 1804) was an 18th-century German philosopher from the Prussian city of Königsberg (now Kaliningrad, Russia). He is regarded as one of the most influential thinkers of modern Europe and of the late Enlightenment.
Kant created a new widespread perspective in philosophy which influenced philosophy through the 21st Century. He also published important works of epistemology, as well as works relevant to religion, law, and history. His most important work is the Critique of Pure Reason, an investigation into the limitations and structure of reason itself. It encompasses an attack on traditional metaphysics and epistemology, and highlights Kant's own contribution to these areas. The other main works of his maturity are the Critique of Practical Reason, which concentrates on ethics, and the Critique of Judgment, which investigates aesthetics and teleology.And now, a few I. Kant quotes from ThinkExist.com:
“Two things awe me most, the starry sky above me and the moral law within me.”
“He who is cruel to animals becomes hard also in his dealings with men. We can judge the heart of a man by his treatment of animals.”
“Immaturity is the incapacity to use one's intelligence without the guidance of another.”
“If man makes himself a worm he must not complain when he is trodden on.”
“Experience without theory is blind, but theory without experience is mere intellectual play.”
“Science is organized knowledge. Wisdom is organized life.”
Cheers! - Jason