“Kant’s lectures can be employed, if at all, only in very peculiarly favorable circumstances, and even then only with the most extreme caution, for the reconstruction of the history of his own development and for the characterization  of his scientific standpoint. [...] [But] their importance for knowledge of his character and for the right appreciation of these individual grounds of belief which always remained the same, and which underly all the published works, inclusive of the critical, can hardly be overestimated. Moreover, the lectures, and the later in particular, appear to possess a certain value even now, on account of their happy formulation of detached thoughts, and as constituting an easy introduction to the Kantian philosophy.”
— Erich Adickes [1896, 579-80]
Anyone using these lecture notes would do well to reflect on their provenance. Even Kant questioned their reliability:
Those of my students who are most capable of grasping everything are just the ones who bother least to take explicit and verbatim notes; rather, they write down only the main points, which they can think over afterwards. Those who are most thorough in note-taking are seldom capable of distinguishing the important from the unimportant. They pile a mass of misunderstood stuff under what they may possibly have grasped correctly. [Letter to Herz, 20 October 1778, AA 10:242][see]
Kant “was not keen on students taking notes. It bothered him when he noticed that more important material was being overlooked, while the less important was being noted down” [Borowski 1804, 187].
Nearly all of Kant’s notetakers were teenagers, of course, and some quite young (the average age of matriculating students was sixteen). They arrived in Kant’s classroom with such widely varying levels of intelligence and academic preparedness that it is quite right to question their ability to provide reliable notes — What could such efforts possibly add to Kant’s own writings? — and they were just as likely to misunderstand Kant as not, whose lectures were known to be quite challenging. Consider this handful of contemporary accounts:
Borowski: “Admittedly it was necessary to pay close attention to his lectures” [see] and “Of course, a lively attentiveness was always required. Without this his lectures couldn’t be understood, and one would get lost” [see].
Mortzfeld: “The opinion had spread even among his students that his lectures were hard to comprehend, for which reason most began with his course on physical geography, or with the moral philosophy” [see].
von Baczko: “When I arrived at the Academy, he was giving public lectures. I attended this lecture and didn’t understand it [...] I noticed that many students in Kant's classroom knew even less than me, and I began to believe that they were attending Kant’s lectures just to show off.” [see].
Jachmann: “Whoever did not understand this way of his would take his first explanation as the correct and fully exhaustive one, and so would not follow him very closely after that, thus collecting mere half-truths — just as several sets of student notes have convinced me. [...] Kant also knew quite well that his philosophy lessons were not easy for the beginner, and thus would openly ask the students to prepare themselves first with Professor Pörschke’s lectures.” [see].
As Kant aged, the difficulty of the lectures, and thus of taking good notes, was a result not just of the subject matter but also of his delivery — something to consider when using the notes from the 1790s:
Reusch: “When I came to the university at Michaelmas 1793, Kant was already in his 70th year, his voice was weak, and he would get himself tangled up in his lectures and become unclear. [...] To a young man of 15-16 years under those circumstances, not much of his philosophical lectures could be put into a context that made them understandable; what I grasped was an occasional illuminating point or spark in the soul. I don’t believe that it went any better at that time with the older students.” [see].
This is why Rosenkranz and Schubert chose to omit the student lecture notes from their 1838-42 edition of Kant’s writings:
[T]hese writings still essentially contain nothing that does not appear in the other writings, or they betray visible traces of foreign matter that were mixed in through a deficient comprehension of the heard lecture and a presumed completion through later interpolation. [see]
 For instance, Graf Dohna [bio], from whom we have notes on logic, metaphysics, anthropology, and physical geography, was barely 14 when he matriculated. The background of these students is briefly discussed here.
 Karl Rosenkranz and Friedrich Wilhelm Schubert, eds. Immanuel Kant’s sämmtliche Werke, 12 vols. (Leipzig: Voss, 1838-42). Hartenstein also omitted the lecture notes in his two editions: Gustav Hartenstein, ed., Immanuel Kant’s Werke, 10 vols. (Leipzig: Modes und Baumann, 11838-39) and Immanuel Kant’s sämmtliche Werke, 8 vols. (Leipzig: Voss, 21867-68). The first edition was arranged thematically while the second was chronological. Hartenstein mentioned the notes in the preface of the 1867-68 edition, and here it is clear that he has in mind only those notes that had been previously published (the two volumes each by Pölitz and Starke):
These writings collectively have so little claim to authenticity that that may remain as disregarded here as they were in the two earlier collected editions of Kant’s works. [1867-68, vol. 1, iii]
Diese Schriften können sämmtlich so wenig auf Authentie Anspruch machen, dass sie hier ebenso, wie in den früheren zwei Gesammtausgaben der Werke Kant’s, unberücksichtigt bleiben durften.
Apart from the worry that the student note-takers might misunderstand the lecture is the fact that many of the notes were the product of a “flourishing branch of industry” [Adickes 1913a, 8] at the university, with “professional” copyists — professional only in the sense that they worked for money — churning out notes for sale to students, almost certainly without ever having stepped foot into Kant’s lecture hall and otherwise untutored in the subject-matter of what they were copying, which opened up their work to the most egregious mistakes, apart from those that always accompany such copying efforts (e.g., the skipping of words and entire lines, misreading proper names, and so on).
Despite Kant’s reservations about the value of the student notes, however, he still took considerable pains to procure copies of notes for various acquaintances: metaphysics, philosophical encyclopedia, anthropology, and possibly logic, for his former student Marcus Herz [AA 10:235-36, 244-45, 245-46, 247], physical geography for the Minister of Culture Karl von Zedlitz [AA 10:222-23, 224-25], both in Berlin. This surely counts as some endorsement of their worth.
Wilhelm Dilthey broke rank with past editors of Kant’s collected writings [Hartenstein 1838-39, 1867-68; Rosenkrantz and Schubert 1838-42] by deciding to include the lectures as the fourth part of the newly conceived critical edition of Kant’s writings sponsored by the Royal Prussian Academy of Sciences (i.e., the Academy edition). He understood the lecture notes to be an important part of Kant’s “intellectual legacy” and, while noting the uncertainty of the transmission of the notes and the need to not regard them as verbatim transcripts of Kant’s spoken words, he found three compelling reasons for their inclusion and study: they provide context for the published writings, they enhance our understanding of Kant’s intellectual development, and they offer us a picture of Kant’s teaching activity and his influence on his students (1902; AA 1:v) [see].
Erich Adickes took a much dimmer view of the prospects offered by the lecture notes (this in the context of his discussion of recent publications by Heinze  and Arnoldt [1890a, etc.] that concerned, primarily, the notes on metaphysics):
I, at least, have come to the conclusion, on the basis of the material now before the world, the investigations of Heinze, and more especially those of Arnoldt, that Kant’s lectures can be employed, if at all, only in very peculiarly favorable circumstances, and even then only with the most extreme caution, for the reconstruction of the history of his own development and for the characterization  of his scientific standpoint. If only one set of notes of a lecture course is preserved, the passages in which it differs from Kant’s published works may, perhaps, be due to a mistake on the part of the writer: he may have taken preliminary views and definitions, which are to be made precise in the course of subsequent lectures, as final, — or the opinions of others, cited by Kant, as Kant’s own opinions, etc. If the notebook cannot be dated with some degree of certainty by external evidence, it is hardly possible to make use of it at all. Views expressed in the older Politz MS. [i.e., an-Pölitz 1], which show more or less of disagreement with the teachings of the Kritik and the other critical works, are repeated in (or at least closely resemble those of) the lectures about 1790. In the rational psychology of the later course, more especially, we find statements which, if undated, would certainly lead us to infer that the paralogisms were as yet undiscovered. Evidently, then, Kant was far more conservative and dogmatic in his lectures than in his books. In the former his chief object was, on the one hand to teach the students to philosophize, to guide them into the right method, and to practise them in critical inquiry, and on the other to confirm them in an ethical-religious conception of the universe. To attain this second end, he attached — in questions relating to God and the mind — a significance to purely theoretical views, reasons and discussions, and to transcendent investigations, with which the logic of his system could not accredit them. Certain personal convictions, partly favorite views of his own, — perhaps the germ of his whole philosophical thinking, — which in the critical period had descended from their former level of authoritative doctrine to that of articles of individual belief (the doctrine of monads, e.g.), still occupy an important place in his lectures. [1896, 579-80; the text was originally published in English]
Adickes closed his discussion on a more positive note for the notes:
Hence however small, for all these reasons, may be the value of the lectures for the reconstruction of the history of Kant’s mental development, their importance for knowledge of his character and for the right appreciation of these individual grounds of belief which always remained the same, and which underly all the published works, inclusive of the critical, can hardly be overestimated. Moreover, the lectures, and the later in particular, appear to possess a certain value even now, on account of their happy formulation of detached thoughts, and as constituting an easy introduction to the Kantian philosophy. [1896, 580]
To summarize the concerns voiced above:
• The lecture notes are always secondary to Kant’s published writings in determining what Kant believed (and here I should add the importance of also consulting the relevant Reflections).
• We must be able to date the source-lecture of the notes — exactly or within some fairly narrow approximation — without which the notes are nearly useless.
• We must not lose sight of how these notes were produced: originally written by students, sometimes copied many times over, often resulting in compilations from more than one source-lecture.
The value of the notes was already discussed in the “Introduction”; their careful study is warranted so long as the above concerns are addressed. Determining how closely a set of notes reflects what Kant said in the classroom is not straight-forward and generally requires a comparison with his published writings, his correspondence, and his own scattered remarks written in his textbooks and other of the Reflexionen, as well as checking consistency with other (independent) sets of notes. And all of this needs to be prefaced by a determination of the semester in which the notes originated and determining how closely the notes stand to that semester. These matters are discussed in the pages on dating the notes and how they were produced. Specifics are considered in the descriptions of the individual sets of notes.
 Although Dilthey did not make this reversal in isolation. The work of Erdmann [1883, 1884], Arnoldt [1890, 1892; reprinted 1908-9], and Heinze  in particular brought positive attention to the notes — here primarily those on metaphysics — and also brought to light newly discovered sets of notes. Vaihinger [1895, 420-21] recounts this shift, and see also the more recent overview of the notes in Sala .
A question preliminary to the question of dating has also been raised: Even if the notes accurately reflect the words that Kant said in the lectures, did Kant actually believe what he said? This question precedes that of dating, since the date of the lecture is rather less important if Kant was merely mouthing the textbook during his many years of teaching. Rosenkranz [1838: vol. 3, vii-viii] first raised this possibility (made in the context of the Jäsche Logic) with his suggestion that Kant was living a “double life”: expressing traditional beliefs in the classroom and censoring his actual beliefs that eventually made their way into his writings [see].
Heinze [1894, 658] remarked how Kant never shook free from the outline of Baumgarten’s Metaphysica, and that the lectures in general share this conservative character, even after the Critique of Pure Reason. Did Kant really believe what he was saying in class? Heinze suggested that Kant was being cautious, but not deceitful, with his students:
I gladly admit that Kant was more cautious in his lectures, out of consideration for strengthening his morally and religiously formative students, than in his published writings, such that he perhaps did not say everything that he held to be true. But I cannot square with his unquestionable and sublime truthfulness that he would say anything other to his students than his own opinion as to what at that moment was his innermost conviction. There is much that certainly sounds quite dogmatic, where he does not always add the critical limitations; but then he was also inwardly drawn to this dogmatic propositions. In general it appears to me that he said directly in his oral lectures what was in his writings, that he lectured to the students that which moved and motivated him most deeply, what was the main intention by his philosophizing, namely, the strengthening of morality and religion. Thus the rational psychology and the theology.
Ich gebe gerne zu, dass Kant mit Rücksicht auf seine zu bildenden, moralisch und religiös zu festigenden Zuhörer in seinen Vorlesungen mehr Vorsicht gebraucht hat, als in seinen veröffentlichten Schriften, so dass er vielleicht nicht immer Alles was er für wahr hielt, vorgetragen haben mag; aber das kann ich mit seiner über allen Zweifel erhabenen Wahrhaftigkeit nicht vereinigen, dass er etwas Anderes seinen Zuhörern scheinbar als seine Meinung kundgab, als was im Augenblick seine innerste Überzeugung war. Manches klingt da allerdings recht dogmatisch, da er die kritische Einschränkung nicht stets beifügt; aber dann neigt er innerlich auch diesen dogmatischen Sätzen zu. Überhaupt scheint es mir, dass er sich in seinem mündlichen Vortrag unmittelbarer giebt, als in seinen Schriften, dass er vor den Studenten das, was ihn am tiefsten bewegte und trieb, was die Hauptabsicht bei seinem Philosophieren war, die Befestigung von Moral und Religion, besonders stark hervortreten liess. Darum die ausgeführte rationale Psychologie, die ausgeführte Theologie. [Heinze 1894, 658]
Heinze then lifted this as another virtue of these student lecture notes: they offer us a glimpse of a Kant never revealed in the published works:
We become better acquainted in the lectures with Kant in his deepest work, in his striving towards something positive, but also in his wobbling, than any view of himself that he offers in his published works. It is not seldom that he airs his heart with some pregnant expression arising from his innermost conviction. Precisely because of this clearer emergence of Kant's whole personality are the lectures not to be undervalued.
Wir lernen ihn so aus den Vorlesungen in seiner innersten Arbeit, in seinem Drange nach etwas Positivem, aber auch in seinem Schwanken besser kennen als aus seinen von ihm selbst heraus gegebenen Werken. Nicht selten kommt ein prägnanter Ausdruck der innersten Überzeugung, durch den er seinem Herzen dann Luft macht, zu Tage. Schon wegen dieses deutlicheren Hervortretens der ganzen Persönlichkeit Kants sind die Vorlesungen von nicht zu unterschätzendem Werthe. [Heinze 1894, 658]
Heinze was responding to comments made by Emil Arnoldt [1892, 402f.; summarized by Vaihinger 1895, 428-29], who believed that Kant was simply accomodating his students in his lectures, but that he did not actually believe some of what he said. Vaihinger [1895, 429], for his part, sides with Heinze, writing that:
two souls lived in Kant’s breast, one negative-critical and one positive-dogmatic, with the latter never entirely overcome by the former. I think that whoever ignores this latter side of Kant will never possess the full and complete Kant.
in Kants Brust wohnten zwei Seelen, eine negativ-kritische und eine positiv-dogmatische, welch letztere durch die erstere niemals ganz überwunden worden ist. Ich meine, dass, wer die letztere Seite bei Kant nicht beachtet, niemals den vollen und ganzen Kant besitzen wird.
Reviewing the metaphysics lectures in AA 28, Rudolf Malter wrote that everything speaks against the thesis of Kant’s “intellectual schizophrenia” [1974, 217-18], and the steady growth over the past thirty-five years in the use of the notes in charting Kant’s intellectual development would seem to put these worries to rest. Kant himself offers testimony against this suggestion of a “double life” in public notice regarding Hippel’s authorship:
[These verbatim passages from the Critique of Pure Reason] gradually flowed as fragments into the notebooks of my students, from my side with respect to a system that I was carrying around in my head, but only in the period from 1770 to 1780 could be brought about. (AA 12:361)
Sie sind nach und nach fragmentarish in die Hefte meiner Zuhörer geflossen, mit Hinsicht, von meiner Seite, auf ein System, was ich in meinem Kopfe trug, aber nur allererst in dem Zeitraume von 1770 bis 1780 zu Stande bringen konnte.
He elaborates on this relationship between his teaching and his published writings in an earlier draft of the notice:
For the university teacher has that advantage over the unaffiliated scholar in his scientific work: that – since with every new course on a subject he must prepare for each lesson (as, to be fair, must always be done) – new views and perspectives always open up to him, partly while preparing, partly – which happens even more often – while in the middle of the presentation, which helps him to correct and expand his sketch from time to time. (AA 13:539; with thanks to Jens Timmermann for translation assistance)
Den Vortheil hat nämlich der Universitätslehrer vor dem zunftfreyen Gelehrten in Bearbeitung der Wissenschaften voraus daß weil er sich bey jedem neuen Cursus derselben auf jede Stunde (wie es billig immer geschehen muß) vorbereiten muß ihm sich immer neue Ansichten und aussichten theils in der Vorbereitung theils welches noch öfterer geschieht mitten in seinem Vortrage eröfnen die ihm dazu dienen seinen Entwurf von Zeit zu Zeit zu berichtigen und zu erweitern. [see]
This claim of Kant’s that parts of his critical system had slipped into his lectures is further confirmed in the memoires of Dirk van Hogendorp, a Dutch soldier and student who attended Kant’s lectures in the early 1780’s:
I do not wish to speak of the philosophical system of this great, splendid man. Few people have grasped it very clearly. [...] But what I can assure you from my own experience, however, is that he developed parts of it with great clarity in his lectures, and that his easily understood expression scarcely required any explanations, that were on occasion asked of him, and which he always gave with the most imaginable helpfulness.
Je ne parlerai pas ici du système philosophique de ce grand et excellent homme. Peu de gens l'ont conçu bien distinctement à travers ce voile d'obscurité répandu sur ses écrits: ce que je puis assurer par expérience,  c'est qu'il en développoit lui-même des fragments dans ses cours avec beaucoup de clarté, et que son élocution facile avoit à. peine besoin des explications qu'on lui demandoit quelquefois et qu'il donnoit toujours avec toute la complaisance imaginable. [Hogendorp 1887, 15-16]