Das ist ein Gleichsetzungsakkusativ, der nur bei Verben mit einem direkten Objekt funktioniert. Dein Beispiel 2 ist daher nicht möglich ("sein" erfordert einen Gleichsetzungsnominativ: "Sie sind ein vernünftiger Mensch"). Also:
Ich kenne Sie als ruhigen Menschen.
Ich halte dich für einen Trottel.
Ich hoffe dich bald gesund wiederzusehen.

In diesem Fall: "Ich habe Sie immer für einen vernünftigen Menschen gehalten" (= Ich kannte Sie als einen vernünftigen Menschen), "aber ich habe mich wohl getäuscht" (= im Augenblick kommen Sie mir sehr unvernünftig vor) -> "Ich glaubte Sie als einen vernünftigen Menschen zu kennen".

Gleichwertig (aber weniger elegant): "Ich glaubte, Sie seien / ugs. wären ...", "Ich habe immer geglaubt, dass Sie ... wären" oder eben "Ich habe Sie immer für ... gehalten".

Engl. etwa: "I have always believed you to be ..." oder "I have known you as ..."; aber "I believed I knew you to be" geht nicht.

I've always known/considered you to be ...

But in English at least, there's a difference in tenses, isn't there?

(1) I believed you to be a reasonable man
(in the past: until today, when you made me doubt that)

vs.

(2) I have (always) believed you to be a reasonable man
(open-ended: so far; please don't force me to doubt that now)

I thought that German 'glaubte' could mean only (1), whereas 'habe geglaubt' would be ambiguous and could mean either (1) or (2).

(1) Ich glaubte, Sie wären ein vernünftiger Mensch.
= Ich glaubte, Sie als einen vernüftigen Menschen zu kennen.
= Ich habe (früher) geglaubt, Sie wären ein vernünftiger Mensch.
(in the past: I thought you were, but you evidently aren't)

vs.

(2a) Ich habe (immer) geglaubt, Sie sind (/seien?) ein vernünftiger Mensch.
= Ich habe (immer) geglaubt, Sie als einen vernünftigen Menschen zu kennen.
(open-ended: so far, and I hope you still are)

That is, since ion1122's sentence has 'glaubte,' not 'habe geglaubt,' it would be simply 'believed' in English, wouldn't it?

To answer the question, yes, as far as I can tell, 'I believed I knew you to be' does seem more literally faithful to Kafka's wording, even though it means basically the same thing as 'I believed you were.'

Thanks everyone. I now have a better understanding of the matter.

To begin with, I now realize that in general “jemanden als etwas kennen” = “to know somebody to be something”:

1. Smith kennt ihn als einen vernüftigen Menschen.

2. Ich kenne ihn als einen vernüftigen Menschen.

I also now realize that an infinitive construction can replace a dass-clause when the subject of the dass-clause is the same as the subject of the main clause:

3. Ich hoffe, dass er sich richtig verhält.

4. Ich hoffe, dass ich mich richtig verhalte.

But instead of 4, also possible (more common?) is:

5a. Ich hoffe, mich richtig zu verhalten.

So then, using the example from Kafka, we have:

6. Ich glaubte, dass ich Sie als einen vernüftigen Menschen kenne/kannte/kennte.

or, more elegantly,

7. Ich glaubte, Sie als einen vernüftigen Menschen zu kennen.

Question: Apparently "glauben" cab be followed by an infinitive as well as a dass-clause. Are all the three following possible?

8. Er glaubt, dass er die Zukunt sieht.
9. Er glaubt, er sieht/sähe die Zukunft.
10. Er glaubt die Zukunft zu sehen

hi ion,

8, 9, 10 are possible, but they will make more sense with "kann". And even more important is the "in":

8. Er glaubt, dass er (in) die Zukunft sehen kann.
9. Er glaubt, er kann (in) die Zukunft sehen.
10. Er glaubt, (in) die Zukunft sehen zu können.

@ ion1122: All your examples are ok, with one exception.
You can't paraphrase Ich glaubte Sie als einen vernünftigen Menschen zu kennen as
"6. Ich glaubte, dass ich Sie als einen vernüftigen Menschen kenne/kannte/kennte." Why?
Both glauben and kennen imply rather permanent convictions / states of mind. When you start the sentence with "ich glaubte" you say that you don't believe anymore, that a conviction belongs to the past and doesn't apply anymore. That makes it difficult to choose the right tense for the verb "kennen" in the dass-clause:

"Ich glaubte, dass ich Sie ... kenne"? No, at the time of speaking I have ceased to know you to be X, so the present tense it out of the question. Could it be the Konkunktiv I? Could be if you consider "glauben" as an utterance of opinion and the dass-clause as reported speech. As the form is indistinguishable from the simple present, writers would avoid it.

"Ich glaubte, dass ich Sie ... kannte"? No, I knew you to be X at the time when I believed it. You either knew him to be X or you believed him to be X, both mean the same thing, and it is unidiomatic to distance yourself from both of them at the same time (unlike English: I thought I knew you, but ...).

"Ich glaubte, dass ich Sie ... kennte"? OK, with a pinch of salt. If you use it without the "to be X", it is something you might say in colloquial speech ("Ich glaubte, dass ich dich kennen würde"), but even that does not work well for "know to be X". Besides, kennte is not colloquial, and, more important, if you use a Konjunktiv II (which raises doubts as to whether the proposition is true) together with a firm expression of conviction such as "glauben", the two of them clash.

All these variants sound rather awkward or incorrect. There are good reasons for using the infinite form "kennen" which is the only one which steers you safely around this calamity.

@ hm--us: I'd like to add a comment on what you said in #3.First, there is no difference of meaning in the German tenses "glaubte" or "habe geglaubt". Not in this sentence, anyway. (There would be a difference between "Ich glaubte an Engel" -- this is a belief in the past -- and "Ich habe (immer) an Engel geglaubt" -- this is something I still believe.) But Ich habe geglaubt, Sie seien ein vernünftiger Mensch and Ich glaubte, Sie seien ein vernünftiger Mensch are virtually indistinguishable. In your example 2a contd. (the construction with the infinitive, "= Ich habe (immer) geglaubt, Sie als einen vernünftigen Menschen zu kennen") the simple past seems preferable to me for stylistic reasons (don't ask me why). But note that the immer is no option in this sentence. It is one of those modal particles which seem superfluous but aren't. In this case, the sense of "immer" is contained in "glauben" and "kennen", both of which impart a sense of permanency. "Ich glaubte immer, Sie als einen vernünftigen Menschen zu kennen" is not idiomatic.But, second, is there really a clear-cut difference in English? It seems rather subtle to me, especially as there is no present perfect contiunuous for "to know". "(2) I have (always) believed you to be a reasonable man(open-ended: so far; please don't force me to doubt that now)"-- yes, but why would you say that in a conversation if not as an expression of doubt? That makes it indistinguishable from your #1 ("in the past: until today, when you made me doubt that"). I agree that #1 ("I believed you to be...") is more definite but the implication (my belief is shaken) is there in #2 as well, and my belief may well end then and there. The German equivalent would be "Ich kenne Sie (doch) als einen vernünftigen Menschen" which gives you a chance to restore my belief in you while "Ich kannte Sie (doch) bisher..." and "Ich glaubte Sie ... zu kennen" make it more and more definite.Third, the various constructions in English and German entail a subtle difference in the degree of conviction involved. Considera) I know him to be a reasonable man = He is a reasonable man, and I know it.b) I know him as a reasonable man = He may or may not be a reasonable man, but we have never experienced anything which suggests that he is not.That's what you find in testimonials: "We have known him as a reliable employee". It means, as far as our experience goes.This second, personal and relative, degree of conviction is expressed in Ich glaubte Sie als einen vernünftigen Menschen zu kennen. The sentence is not composed as(Mr. Samsa is a reasonable man + I know that) + I have reason to doubt this proposition but rather as(Mr. Samsa appeared to me as a reasonable person + he repeatedly gave me this impression and I don't have any evidence to the contrary) + I now have reason to doubt my judgment.Whenever I met you, I got the impression you were a reasonable person; as far as my knowledge of you goes, I never had any reason to believe anything else; but now my belief is shattered and I must admit that my assessment of your personality was sadly wrong.Is this getting too complicated? Oh, poor translators of Kafka's prose!

Terribly sorry. Siehe auch: Formatierungsklau

Here is #7 again, with (hopefully) proper paragraphs:

@ hm--us: I'd like to add a comment on what you said in #3.

First, there is no difference of meaning in the German tenses "glaubte" or "habe geglaubt". Not in this sentence, anyway. (There would be a difference between "Ich glaubte an Engel" -- this is a belief in the past -- and "Ich habe (immer) an Engel geglaubt" -- this is something I still believe.) But Ich habe geglaubt, Sie seien ein vernünftiger Mensch and Ich glaubte, Sie seien ein vernünftiger Mensch are virtually indistinguishable.

In your example 2a contd. (the construction with the infinitive, "= Ich habe (immer) geglaubt, Sie als einen vernünftigen Menschen zu kennen") the simple past seems preferable to me for stylistic reasons (don't ask me why). But note that the immer is no option in this sentence. It is one of those modal particles which seem superfluous but aren't. In this case, the sense of "immer" is contained in "glauben" and "kennen", both of which impart a sense of permanency. "*Ich glaubte immer, Sie als einen vernünftigen Menschen zu kennen" is not idiomatic.

But, second, is there really a clear-cut difference in English? It seems rather subtle to me, especially as there is no present perfect contiunuous for "to know".
"(2) I have (always) believed you to be a reasonable man (open-ended: so far; please don't force me to doubt that now)"-- yes, but why would you say that in a conversation if not as an expression of doubt? That makes it indistinguishable from your #1 ("in the past: until today, when you made me doubt that"). I agree that #1 ("I believed you to be...") is more definite but the implication (my belief is shaken) is there in #2 as well, and my belief may well end then and there. The German equivalent would be "Ich kenne Sie (doch) als einen vernünftigen Menschen", which gives you a chance to restore my belief in you, while "Ich kannte Sie (doch) bisher..." and "Ich glaubte Sie ... zu kennen" make it more and more definite.

Third, the various constructions in English and German entail a subtle difference in the degree of conviction involved. Consider

a) I know him to be a reasonable man = He is a reasonable man, and I know it.
b) I know him as a reasonable man = He may or may not be a reasonable man, but we have never experienced anything which suggests that he is not.
That's what you find in testimonials: "We have known him as a reliable employee". It means, as far as our experience goes.

This second, personal and relative, degree of conviction is expressed in Ich glaubte Sie als einen vernünftigen Menschen zu kennen. The sentence is not composed as
(Mr. Samsa is a reasonable man + I know that) + I have reason to doubt this proposition
but rather as
(Mr. Samsa appeared to me as a reasonable person + he repeatedly gave me this impression and I don't have any evidence to the contrary) + I now have reason to doubt my judgment.

Whenever I met you, I got the impression you were a reasonable person; as far as my knowledge of you goes, I never had any reason to believe anything else; but now my belief is shattered and I must admit that my assessment of your personality was sadly wrong.

Is this getting too complicated? Oh, poor translators of Kafka's prose!

P.S.: I think that Kafka, along with Kleist and Brecht, ranks among the top ten stylists of the German language. You can use them in textbooks.
And I hope the paragraphs (and the italics) are right this time.