There are such limits to our knowledge of nature, must be borne with patience by every sound mind whether he be a scientist or a workman. I can only advise you to leave off all impossible aspirations and trying to conceive things that are beyond conception. This requires a struggle, but a struggle is the lot of many men of all professions.
In: Robert von Helmholtz, Annual Report of the Board of Regents of the Smithsonian Institution, 1889