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— John Stuart Mill"The ends of scientific classification are best answered, when the objects are formed into groups respecting which a greater number of general propositions can be made, and those propositions more important, than could be made respecting any other groups into which the same things could be distributed. ... A classification thus formed is properly scientific or philosophical, and is commonly called a Natural, in contradistinction to a Technical or Artificial, classification or arrangement."
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Extinction has only separated groups: it has by no means made them; for if every form which has ever lived on this earth were suddenly to reappear, though it would be quite impossible to give definitions by which each group could be distinguished from other groups, as all would blend together by steps as fine as those between the finest existing varieties, nevertheless a natural classification, or at least a natural arrangement, would be possible.
— Charles Darwin
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If diphtheria is a disease caused by a microorganism, it is essential that three postulates be fulfilled. The fulfilment of these postulates is necessary in order to demonstrate strictly the parasitic nature of a disease: 1) The organism must be shown to be constantly present in characteristic form and arrangement in the diseased tissue. 2) The organism which, from its behaviour appears to be responsible for the disease, must be isolated and grown in pure culture. 3) The pure culture must be shown to induce the disease experimentally. An early statement of Koch's postulates.
— Friedrich Loeffler
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