'Logic' proved that airplanes can't fly and that H-bombs won't work and that stones don't fall out of the sky. Logic is a way of saying that anything wihich didn't happen yesterday won't happen tomorrow.

A bird is an instrument working according to a mathematical law, which instrument it is within the capacity of man to reproduce, with all its movements.

Flight by machines heavier than air is unpractical and insignificant, if not utterly impossible.

I am well convinced that Aerial Navigation will form a most prominent feature in the progress of civilization. (1804)

If this plane were to crash, we could get a new start on this quasar problem.
Said to colleagues, dramatically cupping his hand over his brow, shortly after the take-off of a propeller plane leaving Austin, Texas, after the Second Texas Symposium for Relativistic Astrophysics in Dec 1964. Various different theories had been presented at the conference. The flight passengers included many of the major scientists in quasar research, including Margaret and Geoffrey Burbridge, Subrahmanyan Chandrasekhar, John Wheeler and Maarten Schmidt.

Let us hope that the advent of a successful flying machine, now only dimly foreseen and nevertheless thought to be possible, will bring nothing but good into the world; that it shall abridge distance, make all parts of the globe accessible, bring men into closer relation with each other, advance civilization, and hasten the promised era in which there shall be nothing but peace and goodwill among all men.

Nothing will ever equal that moment of joyous excitement which filled my whole being when I felt myself flying away from the earth. It was not mere pleasure; it was perfect bliss. Escaped from the frightful torments of persecution and of calumny, I felt that I was answering all in rising above all.

Quite likely the twentieth century is destined to see the natural forces which will enable us to fly from continent to continent with a speed far exceeding that of a bird.

The demonstration that no possible combination of known substances, known forms of machinery and known forms of force, can be united in a practical machine by which men shall fly along distances through the air, seems to the writer as complete as it is possible for the demonstration to be.

The mathematician of to-day admits that he can neither square the circle, duplicate the cube or trisect the angle. May not our mechanicians, in like manner, be ultimately forced to admit that aerial flight is one of that great class of problems with which men can never cope… I do not claim that this is a necessary conclusion from any past experience. But I do think that success must await progress of a different kind from that of invention.
[Written following Samuel Pierpoint Langley's failed attempt to launch his flying machine from a catapult device mounted on a barge in Oct 1903. The Wright Brother's success came on 17 Dec 1903.]

The point [is] largely scientific in character ...[concerning] the methods which can be invented or adopted or discovered to enable the Earth to control the Air, to enable defence from the ground to exercise control—indeed dominance—upon aeroplanes high above its surface. ... science is always able to provide something. We were told that it was impossible to grapple with submarines, but methods were found ... Many things were adopted in war which we were told were technically impossible, but patience, perseverance, and above all the spur of necessity under war conditions, made men's brains act with greater vigour, and science responded to the demands.
[Remarks made in the House of Commons on 7 June 1935. His speculation was later proved correct with the subsequent development of radar during World War II, which was vital in the air defence of Britain.]

The true method of discovery is like the flight of an aeroplane. It starts from the ground of particular observation; it makes a flight in the thin air of imaginative generalization; and it again lands for renewed observation rendered acute by rational interpretation.

To invent an airplane is nothing. To build one is something. But to fly is everything.