6 6 6 D O C U M E N T 4 6 1 M A R C H 1 9 2 9 TRANSLATION Dear Colleague, I am sincerely grateful for your cordial letter of the 13th inst. which I just re- ceived yesterday, for the kind interest you took in my paper,[2] and for having ar- ranged for its presentation to the Berlin Academy, courtesy of Prof. Planck,[3] to whom I ask you, at the earliest opportunity, to convey my thanks and my respects. I will look over the new note mentioned in your letter with obvious interest. Meanwhile, I have become familiar with your point of view with equally great in- terest and I very gladly communicate to you the impression that I have formed of it, in accordance with the wish that you do me the honor of expressing. I am in perfect agreement with you: 1. in aspiring to an even more comprehensive form of the gravitational equa- tions, that eliminates, so to speak, the dualism of the two terms 2. in thinking that, if the world-lattice effectively has a profound importance, such that it turns out to be closely connected (and not simply conveniently) with the physical phenomenon, the same thing I deduced for it must rationally occur for absolute parallelism and for covariant derivation, whereas the path through the Rie- mannian metric would appear to be a useless and damnable complication, made subordinate to the same grid. But for me (not having, and unable to intuit your divinations) it remains shrouded in mystery whether the grid is actually something that reaches the essence of the phenomenon, or only a meaningful model. Setting out on the grounds of agnosticism, lacking creative intuition, I have not spotted in your earlier attempt a new effort to rise to more perfected physical laws, but simply the discovery of a more adequate geometric representation. In such hy- potheses, I have resolved to construct the representation in a minimal way: that is to say, without starting again from the beginning, as you are doing (that leads to the search for a superior scheme) but, more modestly, accepting all of your field equa- tions as definitive (which have already received every possible experimental in- spection), and adding to them the indispensable restriction for identifying one grid, equipped with the necessary requisites. For the rest, I must absolutely expect that my formulation reduces itself to a sim- ple mathematical exercise upon a temporary stage of your conception, a conception that can instead mature on the wings of genius. Meanwhile, I thank you again sincerely for the friendly kindness with which you have honored me. Please accept both my most affectionate greetings and the assur- ance of my fervid admiration, with which I am Very gratefully yours, T. Levi-Civita