'Contradictions may retard, although they cannot ultimately prevent, the recognition of truth,' he once remarked to me when walking in the gardens of the École Normale; 'that is why it is so important to remove the obstacles which temporarily clog and hamper it. In scientific discussions, it is not as in politics,' he added with a smile, 'where demonstration is often difficult. In the natural sciences, doctrines must be based on an assemblage of results, of observations, and of experiments. If a doctrine is challenged, it seldom happens that its truth or falsehood cannot be established by the application of some crucial test. Even a single experiment will often suffice either to refute or consolidate the doctrine.' Reviewing the labours of the past forty years, Pasteur then called to mind the numerous controversies in which he had been engaged. Not only had he been attacked by Pouchet and Joly on the question of spontaneous generation, by Liebig on the subject of fermentation, by Germans and Italians regarding the attenuation of virus, but every one of his assertions had been met with such passionate opposition that, from sheer weariness, he had invariably ended by referring the matter to some authorised commission, only asking it to put an end to all strife by coming to some definite decision. The upshot was at times somewhat amusing. For instance, when Pasteur described to the Academy of Medicine how, simply by lowering the temperature of a hen, he had made her susceptible to inoculation with splenic fever, the facts were at once denied by M. Colin, a professor of the school of Alfort. Pasteur immediately requested that a commission might be named, which should include both himself and his opponent among its members. This was on a Tuesday, one of the Academy days of sitting. The following Saturday, in presence of the whole commission, Pasteur produced four hens that had died of splenic fever. M. Colin himself conducted the autopsy. It was clear to everyone that their blood was full of the filaments of the splenic fever parasite. The _procès-verbal_ was drawn up and signed by all the members of the commission, necessarily including M. Colin. The following Tuesday it was read at the sitting of the Academy. To cover his retreat M. Colin now contended that the hens had taken splenic fever not because they had been subjected to a chilling process, but because, so as to keep them in the water, the poor creatures had had their wings and feet tied to planks. This sentimental objection was disposed of by comparative experiments that had been made on hens similarly tied and inoculated, but not chilled. The latter had in no case taken the disease.