Frederick C. Beiser, in his essay “Maimon’s Critical Philosophy” (Chapter 10; The Fate of Reason: German Philosophy from Kant to Fichte) says that “Kant, who was sixty-six, in failing health, and eager to finish the third Kritik, nearly returned the parcel, citing his age and health as an excuse. After a glance, however, he was so convinced of the quality of the manuscript that he felt compelled to read through several sections and to write a lengthy reply.”
In his May 26, 1789, letter to Hertz, Kant gave this verdict on Maimon’s manuscript: "... but a glance soon enabled me to recognize its merits and to see not only that none of my opponents had understood me so well, but that very few could claim so much penetration and subtlety of mind in profound inquiries of this sort.”
Here’s Beiser’s description of Maimon:
The author of this strange manuscript was himself a very strange character. A Polish-Russian Jew, in fact, a rabbi, he came from the most humble circumstances and was then leading a precarious existence in Berlin. Having never received a university education, his only philosophical training came from the Talmudic tradition. His native language was an almost incomprehensible combination of Hebrew, Lithuanian, Yiddish, and Polish, so that upon his arrival in Berlin only such a skilled linguist as Mendelssohn could understand him. His life had been a long tale of woe: he had lived in constant poverty; he had a broken marriage behind him; he was exiled from his community because of his unorthodoxy; and for several years he was even a wandering beggar. Understandably, he was a man of few social graces. He was crude, naive, and simple, and frequently embarrassingly outspoken in expressing his radical views. Since he often drank away his misery, he spent most of his time in taverns, where he would write his philosophy on wobbly tables, and where anyone could buy his amusing conversation for a few drinks. In short, this character was the Rameau's nephew of eighteenth-century Berlin. But we must not forget: he was also the man whom Kant regarded as his best critic. This was Salomon ben Joshua, or as he liked to call himself in honor of Moses Maimonides, the twelfth-century Spanish-Jewish philosopher, Salomon Maimon.On Maimon’s contribution to Kantian philosophy, Beiser says:
Just as Kant was awakened by Hume's skepticism, so Fichte, Schelling, and Hegel were challenged by Maimon's skepticism. What shook them out of their Kantian slumbers was Maimon's attack upon the transcendental deduction. According to Maimon, the central question behind the deduction—How do synthetic a priori concepts apply to experience?—could not be resolved on Kantian premises. Kant had created such an unbridgeable dualism between understanding and sensibility that it became impossible for synthetic a priori concepts to correspond to experience. This argument created a new and daunting task for Fichte, Schelling, and Hegel: to find a more plausible solution to the problem of the deduction. Like Kant, they were eager to defend the possibility of synthetic a priori knowledge; but they had to admit Maimon's point that such knowledge is not possible given Kant's dualism.In his essay Beiser shows that there is some kind of coherence in Maimon’s philosophical thought. He credits Maimon with developing a critical middle path between skepticism and dogmatism, even though Maimon did not carefully articulate his pivotal ideas. The task of developing the full implications of Maimon’s thought fell on Johann Gottlieb Fichte, Maimon’s great successor.