The Mediator between Brain and Hands must be the Heart.

[Der Mittler zwischen Hirn und Händen muss das Herz sein.]

Thea von Harbou
Thea von Harbou (1888-1954) German screenwriter, novelist, film director, actress
Metropolis, ch. 5 [Maria] (1925) [tr. (1927)]
    (Source)

The novel was written to be the basis for the film by Von Harbou's husband, Fritz Lang. She also collaborated with him on the script. The movie began shooting before the novel was published.

In talking with the growingly restless workers of the city, Maria adds, shortly after the above line:

One will come, who will speak for you -- who will be the mediator between you, the Hands, and the man whose Brain and Will are over you all.

Von Harbau also included an epigraph at the beginning of the novel, which concludes (with a slightly different translation): "The mediator between brain and muscle must be the Heart."