Moses Mendelssohn’s Big Question: Can Reason Prove God?
A Dare from a Pastor
In 1769, a Swiss pastor named Johann Caspar Lavater (1741–1801) put a thick envelope into the hands of Moses Mendelssohn (1729–1786). The letter wasn’t friendly. Lavater demanded that Mendelssohn either refute every argument for Christianity—or convert on the spot. Mendelssohn could have thrown the letter away. Instead, he wrote a calm, clever reply. He refused to attack anyone else’s religion, but he insisted that Judaism could stand on its own, using nothing but reason. The exchange made him a hero across Europe.
Mendelssohn was not a trained university professor. He grew up in the German town of Dessau, followed a rabbi to Berlin, and worked as an accountant for a silk factory. Because he was Jewish, many doors stayed closed. But his friend, the playwright Gotthold Ephraim Lessing (1729–1781), helped him publish anonymous essays that crackled with ideas. In 1763, still a young man, Mendelssohn won the Prussian Royal Academy’s prize contest by arguing that metaphysics—the study of reality’s deepest structure—could be as certain as geometry. He wrote, “The analysis of concepts is for the understanding nothing more than what the magnifying glass is for sight.” Both math and metaphysics break ideas into pieces, he said.
Yet he noticed three reasons metaphysics lagged behind: it depends on artificial words that don’t resemble what they mean, its concepts are so tangled that you can’t define one without knowing all the others, and it must prove that the things it describes actually exist outside our minds. That last hurdle is the problem of idealism—the worry that the whole world might just be a dream. That worry would hound Mendelssohn for the rest of his life.
Can a Body Think? The Immortal Soul
Borrowing the title of Plato’s dialogue, Mendelssohn published his own Phaedo in 1767. The story retells Socrates’s last hours, but Mendelssohn swapped in a new argument for why the soul cannot be destroyed. He called this rational psychology—an attempt to study the soul through concepts and logic rather than by looking at it.
His reasoning was startlingly simple. When you remember your tenth birthday or compare two jokes, you aren’t a cloud of separate thoughts bumping into one another. A single “I” unites them. Mendelssohn put it this way: “We would be able neither to remember nor to reflect nor to compare nor to think, indeed, we would not even be the person who we were a moment ago, if our concepts were divided among many.” If your mind were just a pile of physical parts, nothing would tie your experiences together. So there must be at least one simple substance—something not made of smaller parts—that does the tying.
Because a simple substance cannot be chopped into pieces, your soul is indestructible. The body might decay, but the thinking core survives. Later, Immanuel Kant (1724–1804) would challenge this kind of argument, insisting that “existence” isn’t a feature you can prove just by unpacking a definition. But Mendelssohn’s bestseller convinced thousands of readers that reason could build a ladder to the afterlife.
“I Am, Therefore God Is”: The Logic of the Divine
Mendelssohn never stopped trying to prove that God exists. In his prize essay and again in his final book, Morning Hours (1785), he offered two main routes.
The first begins with a fact you can’t doubt: you are here, but you didn’t have to be. You are a contingent being—something that once did not exist and could have remained nothing. Mendelssohn relied on the Principle of Sufficient Reason, the idea that everything has an explanation. A chain of contingent things can’t explain itself; sooner or later you need a necessary being that exists by its own nature. He summed up the result in a striking slogan: “I am, therefore there is a God.”
The second proof works like a logic puzzle. Imagine the most perfect being that can be thought. If it did not exist, it would be either impossible (the idea contradicts itself) or merely possible (it depends on something else to become real). But a perfect being can’t be dependent—that would make it less perfect. And the idea doesn’t contain a contradiction. Therefore, such a being must actually exist. When critics complained that existence isn’t a property you can list like “red” or “tall,” Mendelssohn pushed back: we can still think contingent things without existence, so existence makes a real difference.
Near the end of Morning Hours, he added a quieter proof from the fact that you know yourself only dimly. “I am not merely what I distinctly know of myself,” he wrote. An infinite mind, he argued, must be grasping all the truths that escape you. For Mendelssohn, reason didn’t have to leave the house to find God—it just had to look carefully within.
Why We Tremble at a Mountain: Beauty and the Sublime
Mendelssohn was one of the first thinkers to give feelings a serious job in philosophy. He defined beauty as a “sensuously perfect representation”—when you sense a harmonious pattern all at once, without dissecting it. Music, painting, and poetry can flood you with a pleasure that sits halfway between raw sensation and cold reasoning.
But why do people pay to watch a tragic story that ends in grief? His answer was the idea of mixed sentiments. A representation has two sides: an objective picture (the sad event) and a subjective effect on your soul. Even when the picture is painful, the act of grasping it can affirm some perfection in you, and that feels good. In a tragedy, you admire the hero’s courage at the same moment your heart breaks. Both streams mix together.
Then there is the sublime—that spine-tingling shudder you feel facing an endless ocean, a storm, or a display of extraordinary genius. Mendelssohn said these experiences involve “intensive immensity,” a greatness that outruns your everyday expectations. You tremble, but you also want to stay. He saw beauty and sublimity as a bridge from knowing to desiring—and that bridge would later inspire Kant to build his own system around aesthetic judgment.
The Sword and the Conscience: How to Be Free
In 1783 Mendelssohn published Jerusalem, a small book that shook the way Europe thought about religious power. He argued that the state has physical force—soldiers, prisons, laws—and can use it to make people act a certain way. But it must never coerce what goes on inside a person’s mind. “The power of religion,” he wrote, “is love and beneficence.” True faith convinces; it does not threaten.
He then turned that idea toward his own tradition. Judaism, he claimed, is not a set of secret supernatural truths whispered from heaven. Reason alone can discover everything essential. Instead, the Torah is a divine legislation—a way of life meant to shape actions through rituals, stories, and community. Because it relies on a living oral tradition, its meaning can shift with new generations without becoming a frozen idol. Mendelssohn worried that written language, once it becomes a sacred object, tempts people to worship words themselves rather than what they point toward.
This vision of Judaism allowed him to champion tolerance without watering down his own identity. He did not say “all religions are the same.” He said that a Jew can remain fully Jewish while respecting everyone else’s conscience—because no one, king or rabbi, may command a belief.
A Dead Friend, a Furious Fight, and a Lesson for Us All
Mendelssohn’s closest friend, Lessing, died in 1781. Soon afterward, a writer named Friedrich Heinrich Jacobi (1743–1819) dropped a bombshell: he claimed Lessing had secretly believed in pantheism—the idea that God and the universe are one single thing. To many, pantheism sounded like atheism with a polite smile. Jacobi argued that pure reason inevitably slides into it, and that only a blind leap of faith could save religion.
Mendelssohn was crushed. He had spent decades showing that reason and faith walk hand in hand. In Morning Hours, he fought back by imagining what a “refined pantheism” might look like: God as the infinite mind that thinks the best possible world into being, much as a novelist holds a whole story in her head. A refined pantheist, he argued, merely says that the world exists as God’s thought. The difference from traditional theism was paper-thin. Mendelssohn wrote, “I fear that, in the end, the famous debate among materialists, idealists, and dualists amounts to a merely verbal dispute that is more a matter for the linguist than for the speculative philosopher.”
That insight—that many explosive arguments are really just tangles of language—is a superpower. Mendelssohn realized that if you can unpack what people actually mean by their words, the battle often vanishes. The lesson didn’t come cheaply. Rushing his final reply to Jacobi through the Berlin winter without an overcoat, Mendelssohn caught a fatal cold and died in January 1786.
Today, when someone on a screen screams that science and religion are at war, or when a disagreement with a friend turns into a shouting match, Mendelssohn’s quiet question still works: What do you really mean by that? Before you take a side, look at the words. Respecting someone else’s conscience doesn’t mean you have to agree. It means you’re willing to do the hardest thing philosophy asks of you—think together.
Think about it
- If you could prove beyond doubt that your soul continues after your body dies, would it change how you treat other people today? Why might it—or why might it not?
- Mendelssohn believed that some of the loudest arguments are actually just “word disputes.” Can you think of a real fight you’ve had where the problem was that you and the other person meant different things by the same word?
- Should a government ever be allowed to punish someone for what they believe, even if they never act on that belief? What could go wrong?
