The question between spirit and matter appears to me nugatory because we have neither evidence nor idea of either. All that we certainly know is that some substance exists, which must be the cause of all the qualitys and Attributes which we perceive: Extension, Solidity, Perception, memory, and Reason, for all these are Attributes, or adjectives, and not Essences or substantives.
Sixty years ago, at College, I read Berkley, and from that time to this I have been fully persuaded that we know nothing of Essences, that some Essence does exist, which causes our minds with all their ideas, and this visible World with all its wonders. I am certain that this Cause is wise, Benevolent and powerful, beyond all conception; I cannot doubt, but what it is, I cannot conjecture.
Suppose we dwell a little on this matter. The Infinite divisibility of it had long ago been demonstrated by Mathematicians--When the Marquis De L'Hospital arose and demonstrated that there were quantities and not infinitely little, but others infinitely less than those infinitely littles, and he might have gone on, for what I know, to all Eternity demonstrating that there are quantities infinitely littles, and he might have gone on, for what I know, to all Eternity demonstrating that there are quantities infinitely less than the last infinitely littles; and the Phenomena of nature seems to coincide with De L'Hospitals demonstrations. For example, Astronomers inform us that the Star draconis is distant from the Earth 38. 000, 000. 000. 000. miles. The Light that proceeds from that Star, therefore, must fill a Sphere of 78. 000, 000, 000, 000, miles in diameter, and every part of that Sphere equal to the size of the pupil of the human Eye. Light is Matter, and every ray, every pencil of that light is made up of particles very little indeed, if not infinitely little, or infinitely less than infinitely little. If this Matter is not fine enough and subtle enough to perceive, to feel and to think, it is too subtle for any human intellect or imagination to conceive, for I defy any human mind to form any idea of anything so small. However, after all, Matter is but Matter; if it is infinitely less than infinitely little, it is incapable of memory, judgement, or feeling, or pleasure or pain, as far as I can conceive. Yet for anything I know, it may be as capable of Sensation and reflection as Spirit, for I confess I know not how Spirit can think, feel or act, any more than Matter. In truth, I cannot conceive how either can move or think, so that I must repose upon your pillow of ignorance, which I find very soft and consoleing, for it absolves my conscience from all culpability in this respect. But I insist upon it that the Saint has as good a right to groan at the Philosopher for asserting that there is nothing but matter in the Universe, As the Philosopher has to laugh at the Saint for saying that there are both Matter and Spirit, Or as the Infidel has to despise Berckley for saying that we cannot prove that there is anything in the Universe but Spirit and Idea--for this indeed is all he asserted, for he never denied the Existence of Matter. After all, I agree that both the groan and the Smile is impertinent, for neither knows what he says, or what he affirms, and I will say of both, as Turgot says of Berkley in his Article of Existence in the Encyclopedia: it is easier to despise than to answer them.
Oh delightful Ignorance! When I arrive at a certainty that I am Ignorant, and that I always must be ignorant, while I live I am happy, for I know I can no longer be responsible.
We shall meet hereafter and laugh at our present botherations. So believes your old Friend,
Jefferson to Adams (Monticello. Aug. 15. 20.)
[L]et me turn to your puzzling letter of May 12. on matter, spirit, motion, etc. It's croud of scepticisms kept me from sleep. I read it, and laid it down: read it, and laid it down, again and again: and to give rest to my mind, I was obliged to recur ultimately to my habitual anodyne, 'I feel: therefore I exist.' I feel bodies which are not myself: there are other existencies then. I call them matter. I feel them changing place. This gives me motion. Where there is an absence of matter, I call it void, or nothing, or immaterial space. On the basis of sensation, of matter and motion, we may erect the fabric of all the certainties we can have or need. I can conceive thought to be an action of a particular organisation of matter, formed for that purpose by it's creator, as well as that attraction is an action of matter, or magnetism of loadstone. When he who denies to the Creator the power of endowing matter with the mode of action called thinking shall shew how he could endow the Sun with the mode of action called attraction, which reins the planets in the tracts of their orbits, or how an absence of matter can have a will, and, by that will, put matter into motion, then the materialist may be lawfully required to explain the process by which matter exercises the faculty of thinking. 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. At what age of the Christian church this heresy of immaterialism, this masked atheism, crept in, I do not know. But a heresy it certainly is. Jesus taught nothing of it. He told us indeed that 'God is spirit,' but he has not defined what a spirit is, nor said that it is not matter. And the antient fathers generally, if not universally, held it to be matter: light and thin indeed, an etherial gas; but still matter. [...] All heresies being now done away with us, these schismatics are merely atheists, differing from the material Atheists only in their belief that 'nothing made something,' and from the material deist who believes that matter alone can operate on matter.
Rejecting all organs of information therefore but my senses, I rid myself of Pyrrhonisms with which an indulgence in speculations hyperphysical and antiphysical so uselessly occupy and disquiet the mind. A single sense may indeed be sometimes deceived, but rarely: and never all our senses together, with the faculty of reasoning. They evidence realities; and there are enough of these for the purposes of life, without plunging into the fathomless abyss of dreams and phantasms. 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. I am sure that I really know many, many, things, and none more surely than that I love you with all my heart, and pray for the continuance of your life until you shall be tired of it yourself.
Though Adams' skepticism is quite obviously right, it is Jefferson's closing lines that are the kicker, for no man will deny that he loves and is loved and that this love is something quite real and independent of the world of mere material. That's not a reasoned argument, just an assertion (an expression of faith) but it is sufficient and just as sufficient now as it was then. And so, ultimately, we all groan with the Saint.