[1789 / 65 yrs.] Kant’s letter of 21 September 1791 to K. L. Reinhold.
“About two years ago my health underwent a sudden drastic change, without any evident cause and without any real illness (other than a cold lasting about three weeks), which quickly changed my appetite for the usual daily indulgences. My physical strength and sensations were unaffected, but my disposition for mental work, even lecturing, suffered greatly. I now can work for only two to three hours in the morning, after which I’m too drowsy (no matter how well I slept the night before). I am compelled to work in intervals which slows it down terribly.” [Source: AA 11: 288]
[1791 July / 67 yrs.] Fichte’s letter to Wenzel.
“His lectures are not as useful as his writings. His weak body seems exhausted from harboring such a great mind. Kant is already very frail, and his memory is starting to fail him.” [Source: Fichte, Gesamtausgabe, III.1.243]
[1796 / 72 yrs.] Jachmann’s biography.
“There was no trace of mental illness in him, nothing but mental weakness that steadily increased. Already eight years ago I found him somewhat changed, although on some days, when his natural functions were going well, his former mental powers shone forth. Since that time, however, the decrease of his strength became more noticeable.” [Source: Jachmann 1800, 196]
[1798 February 7 / 73 yrs.] Poerschke’s letter to Fichte.
“…he is no longer adept at sustained thinking; he lives primarily from the rich store of his memory; but even now he often makes exceptional combinations and sketches. […] withdrawn from all social gatherings, except at the house of his friend Motherby [Source: Fichte Briefwechsel, 1: 589-90]
[1798 September 21 / 74 yrs.] Kant’s letter to Christian Garve.
“My health, as others will have informed you, is less that of a scholar than that of a vegetable – capable of eating, moving about, and sleeping.” [Source: AA 12: 257]
[1798 October 19] Kant’s letter to Kiesewetter.
“The state of my health is of one who is old, not sick, but still an invalid, above all too old to perform any official or public service, who nevertheless feels a small measure of powers in him for finishing the work currently at hand; and with that fill a few remaining gaps and bring to a close the critical project.” [Source: AA 12: 258]
[1799 / 75 yrs.] Wasianski’s 1804 biography.
As early as 1799, when it was still hardly noticeable, he once said in my presence, explaining his weakness: “Gentlemen, I am old  and weak, you must consider me as a child.” [Source: Wasianski 1804, 51-52]
[1799 June 27] In a letter to Robert Motherby Kant writes:
As far as concern the mechanical aspects of life, mine is still good enough for my 75 years – I am able to eat, walk, and sleep without pain. But the pressure in my head makes it difficult to  study, which is after all my proper occupation, and so makes life quite bitter for me. [Source: Schöndörffer 1986, 951-52]
[1800 / 76 yrs.] Jachmann’s biography.
“Four years ago he began to keep notes [Gedankenzettels] for recording the travellers visiting him. On these small sheets of paper he finally wrote down every little thing that he had been told by others or that had occurred to him.” [Source: Jachmann 1800, 196]
[1800 April/May] Jachmann’s biography.
Kant fell and hit his head, after which he rarely left the house. [Source: Jachmann 1800, 215-16]
[1800 July 17] F. T. Rink’s letter to Charles Villers.
[…] Please do not take offence that Kant did not answer your letter; he is old and weak. He answers almost no letter any longer, though he receives them from so many excellent scholars; I almost want to say he  is incapable of answering them. Meanwhile he appreciates you very much and wishes you the best. […] [Source: Vaihinger 1880, 287-88]
[1801 / 77 yrs.] Jachmann’s biography.
“Three years ago I had to inform him about my upcoming change of office and place, but even then he found it so  difficult to remember my new office and the character of my new job that I had to dictate everything to him. Even then he felt it, and perhaps more unpleasantly than when he was weaker, that sometimes his thoughts would just abandon him, and he apologized that thinking and understanding was becoming difficult for him, and that he had to quit the topic being discussed. Thus the power of the greatest thinker gradually diminished to the point of complete mental impotence.” [Source: Jachmann 1800, 196-97]
[1801 June 1] F. T. Rink’s letter to Charles Villers.
[…] Kant’s weakness is increasing dramatically. […] [Source: Vaihinger 1880, 292]
 Hasse’s biography.
And from 1801 on his mental strength was noticeably decreased and his thoughts no longer as orderly as usual; yet bright insights were not unusual, like flashes of lightning passing through his head, which showed his uncommon acumen and were worthy of attention. [Source: Hasse 1804, 8]
[1802 January] Wasianski mentions in passing that, when the new servant was hired, Kant was no longer writing or reading.
”Kant was used to drinking his tea alone […] and pursuing his ideas without being disturbed. Although he was no longer reading or writing, the momentum of that long habit was still strong and he could not tolerate anyone around him without being put into the greatest agitation.” [Source: Hasse 1804, 8]
[1802 October 21] Scheffner to Lüdeke.
Fourteen days ago I ate at Kant’s – My God, what is an old man when he becomes a child! [Source: Warda/Diesch 1916-38, 2: 415]
[1803 March 14] Scheffner to Lüdeke
Last Sunday I had lunch with Kant, who could not put three words together. His appetite, his sleeping, etc., are still good, but he appears to have entirely lost his animam rationalem [rational soul]. He does not appear to notice this loss. [Source: Warda/Diesch 1916-38, 2: 423]
[1803 July 12] A brief notice in the Zeitung für die Elegante Welt (Berlin).
(From Königsberg) – Kant, whom I have visited nearly once a week for many years, has been enfeebled for quite some time. His memory has left him almost completely. Now that his eyes have failed him for reading, he will go to bed before six in the afternoon, even on the longest days. He is in his 80th year and is a complete skeleton, despite the rustic food that he still tolerates, such as white peas with pigs-feet, which he must have at least once a week. An excellent man, who is hardly recognizable outwardly. I wish I had kept a diary about him; it would contain the strangest and most entertaining things. [Source: Zeitung für die Elegante Welt, 12 July 1803 (vol. 3, col. 662)]
[1803 October 8 / 79 yrs.] Kant suffers a debilitating stroke after eating an inordinate quantity of English cheese.
Everything was as it had been when suddenly at 9 o’clock in the morning Kant, who was being led by his sister, suddenly sank senseless to the ground from her arm. The servant was called. Kant seemed to have suffered a stroke. The bed was carried out from the cold bedroom into his warmed study. As soon as he was laid down, the servant hurried to me with the hasty notice that his master was dying. [Source: Wasianski 1804, 175; see also Elsner’s account in Jachmann 1804, 218: ]
[1803 October 23] From a brief notice in the Zeitung für die Elegante Welt (Berlin).
… the greatest thinker of our time reduced almost to the state of a child … [Source: Zeitung für die Elegante Welt, 3 November 1803 (vol. 3, col. 1047)]
[1803 October 27] Scheffner to Lüdeke
Kant is now almost completely without a soul, although he is still alive; he often does not know his own household. [Source: Warda/Diesch 1916-38, 2: 436]
[1803 December] Wasianski’s 1804 biography.
In December 1803, he could hardly write his name. His eyesight was so bad that he could not find his spoon and when I dined with him, I would cut up the food for him, put it on the spoon, and put that in his hand. [Source: Wasianski 1804, 189]