Man has no greater enemy than himself. I have acted contrary to my sentiments and inclination; throughout our whole lives we do what we never intended, and what we proposed to do, we leave undone.

Francesco Petrarca (1304-1374) Italian scholar and poet [a.k.a. Petrarch]

(Attributed)
    (Source)

Quoted in Johann Georg Ritter von Zimmermann, An Examination of the Advantages of Solitude and of Its Operations, ch. 5 (1783) [tr. F.S. (1808)].