When Jacobi, in 1783, launched his attack on the Enlightenment’s central theme—the idea that reason justifies all the essential truths of commonsense, morality and religion— Mendelssohn was unable to furnish strong arguments to support the sovereignty of reason. Mendelssohn created the impression of an increasingly hapless thinker who is torn between common sense and speculation. In answer to Jacobi’s charge that reason always leads to nihilism, Mendelssohn admitted that it sometimes leads to nihilism. Because of his weak arguments, Mendelssohn lost support among the scholars of his period. Kant declared that by sometimes siding with speculation, Mendelssohn had betrayed his own ideal of sovereignty of reason.
It is also noteworthy that when the Pantheism controversy broke out Mendelssohn was too old and frail to get into a contest with Jacobi, let alone a formidable challenger as Kant. In December 1785, Mendelssohn completed his An die Freunde Lessings which was his final answer to Jacobi. He wanted to get this work published quickly in order to bring an end to his bitter arguments with Jacobi. On December 31, 1785, a freezing day in Berlin, Mendelssohn left his house to deliver the manuscript to the publisher. As he was in a hurry he did not put on his overcoat and that turned out to be a fatal mistake. He caught cold and died a few days later, on January 4, 1786.
In his essay, “Mendelssohn and the Pantheism Controversy,” (The Fate of Reason: German Philosophy from Kant to Fichte), Frederick C. Beiser offers the following summary of Mendelssohn’s contribution to the pantheism controversy:
If we look back over his contribution to the pantheism controversy, it is difficult to resist the conclusion that, despite his noble intentions, Mendelssohn had weakened the case for reason more than he had strengthened it. He made the case for reason dependent on the claims of rationalist metaphysics; but these claims were, to say the least, very disputable. He assumed that reason could be a sufficient criterion of truth in metaphysics only if the rationalist theory of judgment were correct; but that theory had serious weaknesses, namely, it could not explain real connection or guarantee conclusions of existential significance. Mendelssohn had also based some central moral and religious beliefs-the beliefs in God, providence, and immortality-upon a priori demonstrations. But these demonstrations were severely criticized by Kant in the first Kritik; and Mendelssohn's failure to reply to Kant in any thorough and rigorous fashion left his entire position exposed. So, in the end, it seemed as if Mendelssohn had imperiled, rather than defended, two fundamental claims of reason: its claim to be a sufficient criterion of truth in metaphysics; and its claim to justify our essential moral and religious beliefs.
Another serious weakness of Mendelssohn's defense of reason was that, at bottom, it failed to address the deeper problem that Jacobi had raised. In summoning the ghost of Spinoza, Jacobi was alluding to the apparent fatalistic and atheistic consequences of modern science. It was indeed these consequences of modern science that so deeply disturbed late eighteenth-century thinkers. Mendelssohn did little to allay these fears, however, with his antique Wolffian-style refutation of Spinoza. For what was at stake was not the geometric demonstrations of Spinoza's system, but the naturalistic spirit behind it.
There was also the nagging suspicion that Mendelssohn had betrayed the very credo he set out to defend. His moral and religious beliefs meant more to him than his reason, which he was willing to abandon should it continue to contradict them. That, at any rate, was the sad lesson to be learned from his method of orientation. It seemed that, when the going got rough, Mendelssohn was really on Jacobi's side. Who, then, was going to defend the cause of reason?
Given Mendelssohn's poor showing, it was crucial that someone else enter into the fray to defend the crumbling authority of reason. A new defense was needed that did not repeat Mendelssohn's mistakes. It would have to separate the case for reason from the claims of metaphysics; it would have to respond to the deeper challenge behind Jacobi's Spinozism; and it would have to take an unambiguous stand in favor of reason. It was the destiny of Kant to undertake just such a defense.