Source: The Romantic Generation (1995), Ch. 4 : Formal Interlude
“The concept of 'tonality' signifies the unified relationship of chords to a central tonic and hence comprises two different assumptions: first, the existence of unifying factors, and second, the existence of, or at least the hypothetical ability to reconstruct, a tonal center.”
Romantische Harmonik und ihre Krise in Wagners Tristan (1920), p. 273.
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Quoted in Reich, Willi (1971). Schoenberg: A Critical Biography, p. 34. Translated by Leo Black.

Joseph Fétis, (1844). Traité complet de la théorie et de la pratique de l'harmonie contenant la doctrine de la science et de l'art, 2d ed., p. 166. Brussels and Paris.

"The Folk Songs of Hungary" in Pro Musica VII (October 1928)
Context: Our peasant music, naturally, is invariably tonal, if not always in the sense that the inflexible major and minor system is tonal. (An "atonal" folk-music, in my opinion, is unthinkable.) Since we depend upon a tonal basis of this kind in our creative work, it is quite self-evident that our works are quite pronouncedly tonal in type. I must admit, however, that there was a time when I thought I was approaching a species of twelve-tone music. Yet even in works of that period the absolute tonal foundation is unmistakable.

The Architecture of Theories (1891)
Context: Three conceptions are perpetually turning up at every point in every theory of logic, and in the most rounded systems they occur in connection with one another. They are conceptions so very broad and consequently indefinite that they are hard to seize and may be easily overlooked. I call them the conceptions of First, Second, Third. First is the conception of being or existing independent of anything else. Second is the conception of being relative to, the conception of reaction with, something else. Third is the conception of mediation, whereby a first and second are brought into relation.
“Music is the tonal analogue of emotive life.”
Feeling and Form, ch. 1, p. 27, Scribner (1953)
The Trouble With Physics: The Rise of String Theory, The Fall of a Science, and What Comes Next (2007)
Prokofiev’s piano sonatas : a guide for the listener and the performer (2008), Prokofiev: His Life and the Evolution of His Musical Language

Source: Attributed in posthumous publications, Einstein and the Poet (1983), p. 139