Everything should be as simple as it can be,The quotation site had ferreted this out,* indeed it was their primary source for the quotation – in this form (Einstein had said similar things, or things in more or less the same form, but we don't seem to have a record of him saying precisely this).
But not simpler.
*Don't go there just yet – you'll spoil the suspense of my own pseudo-scholarly narrative.
Aha, thought I. The quotation can't be found in "Anton Reiser's" Albert Einstein: A Biographical Portrait, the celeb biography LZ translated back in 1930 (work he thought so little of he requested his name be removed from the book as translator). But somehow over the past two decades of doing LZ, I had stumbled upon a contemporaneous formulation (contemporaneous that is to the composition of "A"-12, 1950-51) which LZ almost certainly had read. The composer Roger Sessions, writing in the New York Times (LZ's habitual paper), in a piece entitled "How a 'Difficult' Composer Gets That Way" (January 8, 1950): "I also remember a remark of Albert Einstein, which certainly applies to music. He said, in effect, that everything should be as simple as it can be but not simpler!" (Note: Sessions doesn't claim he's quoting AE verbatim, just "in effect." LZ, on the other hand, translates it into a direct quotation.)
I tried to work this little bit of "sourcing" into my LZ publications for years, & never managed to find the right place for it. I figured it would be my little jewel, my one trouvé. But when I saw the author of the Quote Investigator blog on the trail, I emailed him forthwith with my find, and he promptly incorporated it into his piece.
But that's only where the story gets interesting (interesting, that is, if you're a painfully anal-obsessive textual-critic-type). That author, in turn, emailed me back: the quotation also appears – I'd forgotten – in Prepositions, at the end of Part II of "William Carlos Williams" (page 51): a section dated 1948. Lots of thoughts ran thru my head, first of all that perhaps LZ had told the quotation to Sessions, who then used it in his NYT piece. But there's no record of LZ ever meeting Sessions. And I couldn't find the quotation in any of the letters LZ wrote before 1950. So what gives with this "1948"?
Here's what gives: "William Carlos Williams" actually consists of three widely separated essays LZ put together into a single piece for the 1967 publication of Prepositions: Part I, "A Citation," was written for The Nation in 1958; Part III is a 1928 review of WCW Voyage to Pagany, which was published in 1931 in Hound & Horn as a "postscript" to LZ's big Henry Adams essay. And here's the complicated textual history of Part II:
1) It's first published as "Poetry in a Modern Age" in Poetry magazine 76.3 (June 1950), as a review of Vivienne Koch's William Carlos Williams. There are 2 manuscripts and a typescript extant, the middle one dated 19 March 1950.
2) A shorter version is published in Winter 1962 in The Massachusetts Review as "An Old Note on William Carlos Williams," with a date at the end saying "1948." And this version is identical to ––
3) Part II of Prepositions's "William Carlos Williams," which is also dated 1948.
Marcella Booth's scrupulous Catalogue of the LZ Manuscript Collection (1975) lists the manuscript/typescript materials of (1) and (2) as two separate items, dating (1) to 1950 and (2) to 1948, sensibly concluding that LZ incorporated "all the material" in (2) into (1).
Well, it might be sensible to conclude that, but that's not how LZ worked. Time & again, he would reprint a previously printed piece in an abbreviated form: you see it spectacularly in "An Objective," which gives us the high points of all three of the "Objectivist" essays in a single concentrated pill. The magazine publication of his Henry Adams thesis is considerably shorter than the full-length thing at Columbia. So it makes no sense that he would write a short piece on WCW in 1948 – without telling WCW about it – there's no epistolary evidence of his writing it at the time – then pump that up to make a review of Vivienne Koch's book (a book about which he & WCW have significant correspondence).
Here's what happened: LZ wrote a review of Koch in early 1950, making use of the Einstein "quotation" he'd read in the Times, along with a bunch of other things that were obsessing him, & that would similarly appear in "A"-12. A decade later, when a Mass Review editor hit him up for something for a "gathering" for WCW, he sent a new typescript of the piece, cut down by about a third (removing most of the references to Koch's book), and dated the thing – erroneously, it turns out – 1948. And that's the date that stuck when he came to compile Prepositions a few years later.
Don't ask me how much time I spent on this today; it's embarrassing. But I'm heady with the sense of having ironed out a real live error, the sort of thing that gets the textual critic-biographer's pulse racing.