Mr. Potter considers that flight is impractical because materials could not stand the strain – the weight being so great and the material too fragile in proportion to motive power (he compares a flea and an ape). But given materials of sufficient strength he says he can imagine the possibility of flight. He is interested too in the possibility of submarine navigation and has a conception of how it could be done, providing the water under the surface is calm (he envisages a pipe from the surface to the boat, aided by bellows). He has also sent me a pencil sketch of an ingenious cart with legs.John Aubrey: My Own Life. By Ruth Scurr. pg 89, 90
Before the future exists, people have to imagine it. Contemplate it. Tinker with it. Amazing that two friends in the 1600s not only dreamed of flight and submarines, but also sketched and thought through the physical construction that could make it reality.