I recall hearing in yeshiva that one great rabbi -- I think one of the baalei mussar? once said:
If a horse had the mind of Kant, it would author volume after volume of great philosphical works -- all about that a horse should eat oats.
(The point is that halacha is focused on an external source of ultimate truth, whereas Kant was all about innate moral sense.)
Can anyone identify who said this, please?
There is such a story, not regarding philosopher Immanuel Kant, but regarding Prussian (German) statesman Otto von Bismark. Perhaps, the rabbi in yeshiva adapted the story when seeing that his talmidim show interest in philosophy. Certainly, the story would not be significantly different from using Kant instead of Bismark.
The source of the story, Rav Moshe Rosenstein, Sefer Yesodei Da'as, Choveres Aleph, perek 26 (page 40). The mashal is given in connection with his discussion of the differences between chazal and wise men of the world discussed in section 25:
והמשל בזה, אם ינתן לסוס שכלו של ביסמארק אשר היה בזמנו חכם מדיני היותר גדול בעולם, אבל נפש הסוס תשאר כמו שהיתה, הנה לא יהיה בזה לאדם וישאר סוס כמו שהיה, ושכלו הגדול ישתמש בו אך ורק להוציא לפועל עניניו הסוסיים, אף שבעניניו אלו יהיה יותר פקח מכל בני אדם ואם יבוא במשא ומתן עמהם לא יעמדו לפניו כנגד כל תחבולותיו וערמתו, ויוכל לרמות בתחבולות שונות אף את היותר פקחים בבני אדם, ובכל זה ישאר סוס כמו שהית מפני שנפשו היא סוסית, וכל עניניו ושאיפותיו כלם יהיו ענינים סוסיים לבד ולא אנושיים. ועל דרך זה יש לנו לדעת שהבדל מדרגות בני אדם אינו תלוי בגדל השכל והפקחות, כי אם בחשיבות ומעלת לבו ונפשו של אדם, אם המה זכים וטהורים גם השכל זך, ואם נפשו של אדם אינה זכה וטהורה והיא נמשכת אחר הקנאה והתאוה והכבוד גם שכלו ימשך אחריה, ויאמת השקר ויכזב את האמת, ויאמר לרע טוב ולטוב רע לחשך אור ולאור חשך (כמאמר ספר הישר, ש"השכל פרי הנפש וכאשר העץ כן פריו״). וזה עיקר הסוד והיסוד של ההבדל בין חכמינו ז״ל לחכמי העולם, שחז״ל היו קדושי עליון כמלאכי אלהים, ונפשם היתר, זכה ובהירה ומאירה כשמש בצהרים וכאור שבעת ימי בראשית, ובכן היה שכלם זך ובהיר וראו כל האמתיות וידעו סודות העליונים ותחתונים, וכמאמר הכתוב ,סוד ה׳ ליראיו״ , ועליהם יש לנו לסמוך בכל עניני חיינו, אבל חכמי העולם אשר יחסר להם עבודת אלהים ולמוד התורד, וכל המ״ח דברים שהתורה נקנית בהם, הנת היתה נפשם לעומת חז״ל כנפש הסוס לעומת נפשנו, ושכלם בודאי היה נמשך אחרי נפשם מפני שהוא פריה, ומזה ידע כל אחד, שאין שום מקום ודמיון להשוות חכמי העולם לחכמינו ז״ל.
And the parable for it (i.e. comparison of chakhamim z"l with the wise men of the world) is this. If a horse were given the mind of Bismarck, who was in his time the greatest statesman in the world, but the soul of the horse remained equine, as it was, then it would not make him human, rather he would remain being a horse. And its great mind would be used solely to carry out its equine affairs, although in these matters it would be more clever than any human being. And if it entered into negotiations with them, they would not stand before it against all its tricks and deceit. And it would be able to deceive even the cleverest of humans with various tricks, and in all this the horse would remain as it was, because its soul is that of a horse, and all its affairs and aspirations would be solely equine affairs and not human. (The parable suggests an idea for contemplation which helps to understand) that the difference in the spiritual levels of human beings does not depend on the greatness of intellect and intelligence, but on the significance and exaltation of a person's heart and soul. If they are clear and pure, the intellect is also pure. However, if a person's soul is not clear and pure and is drawn to envy, lust, and honor, his mind will also be drawn to it. He will make the lie true and deny the truth, and will call evil good and good evil, darkness light and light darkness (as the Sefer haYashar says, "Intellect is the product (literally "fruit") of the soul, and its fruit is in accordance with the kind of the tree that it is"). This is the main secret and foundation of the difference between our Sages, of blessed memory, and the wise of the world. Our Sages were saints of the Most High, like the angels of G-d, and their souls were pure and bright, they shone like the sun at noon and like the light of the seven days of Creation. Thus, their minds were clear and pure, and perceived eternal truth and knew the secrets of the higher and lower worlds, as the verse Tehilim 25:14 says "The counsel of Hashem is for those who fear Him; to them He makes known His covenant." And we must trust them in all the matters of our lives. However, the wise men of the world lack the service of G-d and the study of the Torah, and all the forty-eight ways in which the Torah is acquired (Pirkei Avos 6:6). Therefore, comparison of their souls to our sages souls would be similar comparison of the soul of a horse compared to our (human) souls. And their intellect would certainly be drawn after their souls because their intellect is the product ("fruit") of their souls. And from this parable everyone understood that there is no way or possibility to compare the wise men of the world to our Sages, of blessed memory.
The anecdote sounds like a simplified version of a parable by the sixth Lubavitcher rebbe, R. Yosef Yitzchak Schneersohn. At a farbrengen in 1985, R. Menachem Mendel Schneerson, the succeeding rebbe, included the teaching in his talk (and appears in Sichos, vol. 27):
The Previous Rebbe told a parable of a coachman who was driving several scholars in his stagecoach. During the trip the scholars were discussing lofty intellectual concepts, and consequently there were three distinct “thoughts” on the trip: The horses were “thinking” of the hay that they would receive when they reach their destination, the coachman’s mind was occupied with the thought of the handsome fee he would collect for the trip, and the scholars were involved with their intellectual pursuits. “Now,” asks the Previous Rebbe, “just because the horse thinks of fodder does that change anything in the intellectual involvement of the scholars?”
Actually this parable could do with some further elucidation. There really is some connection between the thoughts of the horse and the sages. Being en route, their minds are not relaxed and sharp enough for them to reach a decisive halachic conclusion. The Gemara says: “Halachah needs a clear mind” (Eruvin 65a); while they are traveling their minds are not clear.
If so, the horse’s mind is not so insignificant, for the scholars want to conclude their intellectual exercise with a conclusive ruling. This will only happen when they reach their destination, which depends on the horse, which runs along because it “thinks” of the hay that lies ahead! Thus the goal of the sages’ “thought” in the carriage depends on the goal of the horse’s “thought” while running. In this manner they will reach their destination, and make their ruling, which will lead to: “Study is greater for it leads to action” (Kiddushin 40b). And the action will reveal the G‑dly will, which will effect an abode for G‑d in the lower worlds!
Many folktales transform considerably over time as they are transmitted. The lesson you surmise from the short teaching contrasts from the hassidic teaching here, but they share the bare bones. If your anecdote derives from this parable, it's not hard to see how hay becomes oats, and how a relationship between the mind of a horse and Jewish scholars transforms into an equivalence between a horse and a notable German philosopher.
