When once we quit the basis of sensation, all is in the wind. To talk of immaterial existences is to talk of nothings. To say that the human soul, angels, god, are immaterial, is to say they are nothings, or that there is no god, no angels, no soul. I cannot reason otherwise: but I believe I am supported in my creed of materialism by Locke, Tracy, and Stewart.{Letter to John Adams, from Monticello, 15 August 1820}

Question with boldness even the existence of a God; because, if there be one, he must more approve of the homage of reason, than that of blind-folded fear.

And the day will come when the mystical generation of Jesus, by the supreme being as his father in the womb of a virgin will be classed with the fable of the generation of Minerva in the brain of Jupiter. But we may hope that the dawn of reason and freedom of thought in these United States will do away all this artificial scaffolding...{Letter to John Adams, April 11, 1823}

Reason and free inquiry are the only effectual agents against error.

In a republican nation, whose citizens are to be led by reason and persuasion and not by force, the art of reasoning becomes of first importance

I never yet saw an instance of one of two disputants convincing the other by argument.