Hermann Helmholtz's ideas on the lawful orderliness of mental imagery produced by external objects:

(From 'Die neuren Fortschritte in der Theorie des Sehens,' in Vortrage and Reden, 2 Vols., Braunschweig, 1884, 5th edition, Braunschweig, 1903, Vol. 2, pp. 265-365, tranlated in Russel Kahl, ed., Selected Writings of Hermann von Helmholtz, Middletown, Connecticut: Wesleyan University Press, 1971, pp. 144-222, especially pp. 185-186.)
"The excitation of the nerves in the brain and the ideas in our consciousness can be considered images of processes in the external world insofar as the former parallel the latter, that is, insofar as they represent the similarity of objects by a similarity of signs and thus represent a lawful order by a lawful order."

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