Science is not interested to explain a single phenomenon but a class of phenomena by formulating a general principle.
It is not sufficient that a hypothesis explains some isolated phenomena but should provide an adequate explanation to all cases of the similar type. If it is observed that a particular hypothesis is able to explain the phenomenon under investigation we infer that it will be the explanation for all similar cases.
That is on the basis of observation of particular cases we make a generalization that the same is true in all cases of the similar type. If in some cases it is observed that the presence of bacteria results in some unusual symptom, it is assumed that wherever this symptom is found, the bacteria must be there.
So generalization forms the basis of scientific explanation. This also results in the advancement of knowledge for we pass from the known or observed instances to unknown and unobserved cases.
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Mill has attached great importance to generalization for he thinks that induction becomes complete at the stage of generalization. He maintains that the experimental methods conclusively establish a causal relation so the methods help for a generalization.
But inductive generalizations can never have absolute certainty but can have high degree of probability. For the possibility of a generalization being false in face of new facts cannot be ruled out. But generalization constitutes one of the stages of inductive procedure.