— Louis Pasteur
, Context: You say that, in the present state of science, it is wiser to have no opinion: well, I have an opinion, not a sentimental one, but a rational one, having acquired a right to it by twenty years of assiduous labour, and it would be wise in every impartial mind to share it. My opinion — nay more, my conviction — is that, in the present state of science, as you rightly say, spontaneous generation is a chimera; and it would be impossible for you to contradict me, for my experiments all stand forth to prove that spontaneous generation is a chimera. What is then your judgment on my experiments? Have I not a hundred times placed organic matter in contact with pure air in the best conditions for it to produce life spontaneously? Have I not practised on these organic materia which are most favourable, according to all accounts, to the genesis of spontaneity, such as blood, urine, and grape juice? How is it that you do not see the essential difference between my opponents and myself? Not only have I contradicted, proof in hand, every one of their assertions, while they have never dared to seriously contradict one of mine, but, for them, every cause of error benefits their opinion. For me, affirming as I do that there are no spontaneous fermentations, I am bound to eliminate every cause of error, every perturbing influence, I can maintain my results only by means of most irreproachable experiments; their opinions, on the contrary, profit by every insufficient experiment and that is where they find their support.
Original in French from Œuvres de Pasteur, Volume 7 (1939), Masson et cie, p. 539: Mon opinion, mieux encore, ma conviction, c'est que, dans l'état actuel de la science, comme vous dites avec raison, la génération spontanée est une chimère, et il vous serait impossible de me contredire, car mes expériences sont toutes debout, et toutes prouvent que la génération spontanée est une chimère