D O C . 4 3 8 T O S P A N I S H A C A D E M Y O F S C I E N C E S 3 7 5 Your words, dear Mr. Cabrera,[2] have touched the very bottom of my heart, not because they convey for me the honor of this great recognition, but because they demonstrate how deliberately and affectionately you have studied my life’s work, echoing the poet’s words: “We wish to receive less praise and, instead, may we be read industriously.” You also touched upon the weak point of the theory of light quanta, an arduous subject for our generation of physicists. I believe that those dif- ficulties can only be overcome by a theory that not only fundamentally modifies the principle of energy, but perhaps also expands that of causality.[3] Just a while ago, Tetrode has pointed precisely to such possibilities.[4] Even though the princi- ples for the solution to this basic problem have not yet become clear, nevertheless the new impetus towards the unification of all the forces of nature, born in the cra- dle of the theory of relativity, promises satisfactory results. The method employed in this venture is purely mathematico-speculative, characteristic of Levi-Civita, Weyl, Eddington.[5] In this way one can completely relieve the foundation of phys- ics from the disturbing dualism summed up by those two words, gravitation and electricity. I have found the words you have spoken very significant – a reflection of your optimistic hope in the development of science in Spain. Moments of active partic- ipation in the global progress of understanding depend upon external conditions that have now been realized in your country. I believe that a tormented and imper- iled Europe can turn its eyes full of hope towards this people, which is now heading down the road to scientific work after having produced such grand things in the arts for humanity. 439. From Michael I. Pupin[1] [New York,] 4 March 1923 [Not selected for translation.] 440. From Carl Brinkmann Berlin-Grunewald, 9 March 1923 [Not selected for translation.]