Quoted by E.H. Luchins and A.S. Luchins, 'Introduction to the Einstein-Wertheimer Correspondence', Methodology and Science 12 (3) (1979), pp. 165-202.
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Translation from the German by Luchins and Luchins:

'Dear Mr. Einstein,

Many cordial greetings! (- How much I owuld rather prefer to say hello to you once again in person -!) I wish you very happy days in Holland!

Today I have to write to you a few words concerning the - strange - Halle matter. My very dear Mr. Einstein - well, what are these people up to?! And into what awkward position have you, in your endless kindness, allowed these people to put you?!

In April, in Prague, I heard of the "coming significant conference on Einstein, where Prof. Kraus (!) will be allotted the leading role, who will now (finally) uncover, in full publicity, the elementary absurdities of Einstein's theories from the philosophical judgement-seat, so that now it will become clear, how..."

And here I find an invitation of the Kant Society: Mr. Einstein will be in Halle too! Then a publication, a supplementary to the Kant Studies, by somebody, which now in its tendency, on the other hand seems characteristic for the Kant-circles. They would, it seems, like to take about the following position unctously: interesting ideas, which, however, cannot collide in any way with the really philosophical problems. (There is a tendency of a somewhat anxious detachment: "everything in Kant is right" and - wherever possible - exploitation for themselves -)

Now, among other things, the Vossische Newspaper [Zeitung] states: "Following the general meeting of the Kant Society... a number of eminent German scholars, among them Einstein, Abderhalden, Kraus! - Prag, Vaihinger, invite to a scientific forum about the positive idealism in the direction of the philosophy of the As-If" in Halle - Prof. Einstein will participate in the proceedings. And I get a printed invitation: "Some members of the Kant Society, who are interested in the position idealism in the direction of the philosophy of As-If -. The undersigned, some of them members, some not members, of the Kant Society invite herewith to this forum... Also the members of the Kant Society have the right to participate in this As-If Conference. Signed: Abderhalden, Becker, Bergmann, Einstein!, Feldkeller, Fliess, Gocht, Knopf, Koffka, Kowalewski, Kraus-Prag, Müller-Freienfels, R. Schmidt, I. Schultzem Vaihinger, Wichmann, H.Wolf." -!!

For God's sake, in what kind of advertising schemes are you placed there?! Would any physicists of a comparable caliber dare something of this kind? For the largest part fainthearted, lazily ruminating, carping mediocrity and in part like Kraus: impertinent: and in such a publicity-mania - Yes, for heavens sake, if one could at least believe that it would have, after all, any sense whatsoever, that anything at all could be seriously accomplished in such a "Conference", or even only just seriously discussed - but you, you good human being [Guter], do you not know what these people are like and where they want to go?! And even if it were only the indescribable insipidness of these people... With people who aim at serious matters - for example, Cassirer - it would be perfectly possible, but even with such persons better not in such surroundings [Corona]! And what shape could the "Conference" take? The people will deliver their stuff in their characteristic, psychologically habitual way and will performtheir tricky feats of disputation - and you will say a few kind words, and then smile a bit and keep silent - and the people - brrr... That is not a good thing and no good can be expected from it.

That's the way it is with these philosophers - And then there comes for once one like you - and what do these people do with that -! Good God!

(If you really plan to go there, then I would like to come along.) - Well; I had to get that off my chest [soul, Seule], and I cannot yet truthfully imagine at all that you will really go there (a sunny day in the woods or a lecture for workers instead would be something much more beautiful and better) - and now I send you, you good man, you much too good man, my very warm regards and am angry that I had to write to you just such stuff!

All good wishes!

Yours, Wertheimer.' (173-174)

Quoted by J. Canales, The Physicist and the Philosopher: Einstein, Bergson, and the Debate That Changed Our Understanding of Time (Princeton and Oxford: Princeton University Press, 2015).
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'Max Wertheimer, one of the founders of Gestalt psychology, wrote a letter to Einstein warning him how the Kant Society wanted to invite him to Halle "to uncover in public before a philosophical tribunal the elementary absurdities of Einst[einian] theories [note: 'Max Wertheimer to Einstein, 15 May 1920, Berlin']. He recommended that Einstein not go. Wertheimer warned him that the philosopher Oskar Kraus was poised to attack him. Kraus and others would use Einstein's theory of relativity to prove the main points of the Philosophy of As If, which had been developed years earlier by the philosopher Hans Vaihinger [note: 'Max Wertheimer to Einstein, 15 May 1920, Berlin']. According to its proponents, scientists worked under the assumption that theories matched perfectly well with reality, but this belief, in the end, was unwarranted.

Wertheimer thought that [Ernst] Cassirer was different from almost all of the other philosophers in Germany. Cassirer, explained Wertheimer to Einstein, was a real ally. Offering an otherwise almost wholesale indictment of German philosophers, he referred to Cassirer alone as a unique person of "earnest intention."

Einstein paid attention... And he thanked Wertheimer: "It is very nice of you not to let me fall into the trap." [note: 'Einstein to Max Wertheimer, [21 May 1919] [Leyden].' [sic. orig.]]  His wife agreed; "How glad I am that you aren't traveling to Halle! All that fuming, for what purpose? You won't convince that sort anyway." [note: Elsa Einstein to Einstein, [24 May 1920], [Berlin].']  Thanks to Wertheimer and his wife's warnings about "the trap" coming from "that sort" of people, Einstein succeeded in distancing himself from antagonistic philosophers and drawing closer to Cassirer, a person of "earnest intention."' (132-133)