“The erotic instinct is something questionable, and will always be so whatever a future set of laws may have to say on the matter. It belongs, on the one hand, to the original animal nature of man, which will exist as long as man has an animal body. On the other hand, it is connected with the highest forms of the spirit. But it blooms only when the spirit and instinct are in true harmony. If one or the other aspect is missing, then an injury occurs, or at least there is a one-sided lack of balance which easily slips into the pathological. Too much of the animal disfigures the civilized human being, too much culture makes a sick animal.”
The Psychology of the Unconscious (1943)
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C.G. Jung 257
Swiss psychiatrist and psychotherapist who founded analytic… 1875–1961Related quotes

1840s, Letters from New York (1843)
Source: Letters from New York http://www.bartleby.com/66/63/12263.html, vol. 1, letter 18

Interview by Andrea Di Marcantonio

volume I, chapter III: "Comparison of the Mental Powers of Man and the Lower Animals — continued", pages 100-101 http://darwin-online.org.uk/content/frameset?pageseq=113&itemID=F937.1&viewtype=image
The Descent of Man (1871)
Context: As man advances in civilisation, and small tribes are united into larger communities, the simplest reason would tell each individual that he ought to extend his social instincts and sympathies to all the members of the same nation, though personally unknown to him. This point being once reached, there is only an artificial barrier to prevent his sympathies extending to the men of all nations and races. If, indeed, such men are separated from him by great differences in appearance or habits, experience unfortunately shews us how long it is before we look at them as our fellow-creatures. Sympathy beyond the confines of man, that is humanity to the lower animals, seems to be one of the latest moral acquisitions. It is apparently unfelt by savages, except towards their pets. How little the old Romans knew of it is shewn by their abhorrent gladiatorial exhibitions. The very idea of humanity, as far as I could observe, was new to most of the Gauchos of the Pampas. This virtue, one of the noblest with which man is endowed, seems to arise incidentally from our sympathies becoming more tender and more widely diffused, until they are extended to all sentient beings. As soon as this virtue is honoured and practised by some few men, it spreads through instruction and example to the young, and eventually through public opinion.

Third Thesis
Idea for a Universal History from a Cosmopolitan Point of View (1784)

Preface to Villa Rubein and Other Stories (1923)
As quoted by Michio Kaku in Hyperspace (Oxford University Press, 1994), p. 12. ISBN 0-385-47705-8.

Source: 1910s, Proposed Roads To Freedom (1918), Ch. VI: International relations, p. 97