“A mind too active is no mind at all.”
Source: The Selected Letters of Theodore Roethke
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Theodore Roethke 86
American poet 1908–1963Related quotes

Source: Dictionary of Burning Words of Brilliant Writers (1895), P. 5.

“To an active mind, indolence is more painful than labor.”

The Interpretation of Dreams (1900), from The Standard Edition of the Complete Psychological Works of Sigmund Freud, translated by James Strachey.
At any rate the interpretation of dreams is the via regia to a knowledge of the unconscious in the psychic life.
Alternate translation by Abraham Arden Brill, p. 483 http://books.google.com/books?id=OSYJAAAAIAAJ&pg=PA483#v=onepage&q&f=false. Freud did use the Latin phrase via regia in the original as opposed to translating it into the German of the surrounding text.
"Royal road" or via regia is an allusion to a statement attributed to Euclid.
1900s

Source: Treatise Concerning Eternal and Immutable Morality (1731), Ch. 1, sct. 1

“One ruins the mind with too much writing. — One rusts it by not writing at all.”

Source: The Lost World of Thomas Jefferson (1948), Ch. 3, Part 2: The Happy Variety of Minds, p. 125.

Patanjali, in East of existentialism: the Tao of the West http://books.google.co.in/books?id=2WyyAAAAIAAJ, p. 266.
Source: Hegel's Idealism: The Satisfactions of Self-Consciousness (1989), p. 20

“Either mathematics is too big for the human mind, or the human mind is more than a machine.”
As quoted in Topoi : The Categorial Analysis of Logic (1979) by Robert Goldblatt, p. 13