“It is a ridiculous thing for a man not to fly from his own badness, which is indeed possible, but to fly from other men's badness, which is impossible.”

Source: Meditations (c. AD 121–180), Book VIII, 48

VIII, 48
Meditations (c. 121–180 AD), Book VIII
Context: The mind which is free from passions is a citadel, for man has nothing more secure to which he can fly for refuge and for the future be inexpugnable. He then who has not seen this is an ignorant man: but he who has seen it and does not fly to this refuge is unhappy.

Source: Sculpting in Time (1986), p. 239

Civil-suit deposition against the Herring-Curtiss Company (1909), reported in The Dayton News (31 May 1912) http://home.dayton.lib.oh.us/archives/wbcollection/wbscrapbooks1/WBScrapbooks10079.html
Context: My brother and I became seriously interested in the problem of human flight in 1899... We knew that men had by common consent adopted human flight as the standard of impossibility. When a man said, “It can’t be done; a man might as well try to fly,” he was understood as expressing the final limit of impossibility. Our own growing belief that man might nevertheless learn to fly was based on the idea that while thousands of the most dissimilar body structures, such as insects, fish, reptiles, birds and mammals, were flying every day at pleasure, it was reasonable to suppose that man might also fly... We accordingly decided to write to the Smithsonian Institution and inquire for the best books relating to the subject.... Contrary to our previous impression, we found that men of the very highest standing in the profession of science and invention had attempted to solve the problem... But one by one, they had been compelled to confess themselves beaten, and had discontinued their efforts. In studying their failures we found many points of interest to us.
At that time there was no flying art in the proper sense of the word, but only a flying problem. Thousands of men had thought about flying machines and a few had even built machines which they called flying machines, but these were guilty of almost everything except flying. Thousands of pages had been written on the so-called science of flying, but for the most part the ideas set forth, like the designs for machines, were mere speculations and probably ninety per cent was false. Consequently those who tried to study the science of aerodynamics knew not what to believe and what not to believe. Things which seemed reasonable were often found to be untrue, and things which seemed unreasonable were sometimes true. Under this condition of affairs students were accustomed to pay little attention to things that they had not personally tested.

"The Fly"
Lyrics, Achtung Baby (1991)

Derived from a longer quote in Henry V, reported in Josiah Hotchkiss Gilbert, Dictionary of Burning Words of Brilliant Writers (1895), p. 283.
Misattributed

Daedalus or Science and the Future (1923)

Source: An Essay on Old Age, 1732, p. 136