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. At what age of the Christian church this heresy of immaterialism, this masked atheism, crept in, I do not know. But heresy it certainly is. [...] I am satisfied, and sufficiently occupied with the things which are, without tormenting or troubling myself about those which may indeed be, but of which I have no evidence.

Letter to John Adams (15 August 1820).