Definition is an important scientific and philosophical method. In all kinds of scientific and philosophical inquiries definition is provided to make clear the characteristics of the things under investigation. Definition in this sense, sometimes called real definition, should state the essence of the thing defined, according to Aristotle. In another (currently popular) sense, sometimes called nominal definition, definition explicates the meaning of a term already in use in an ordinary language or the scientific discourse or specifies the meaning of a new term introduced in an ordinary language of the scientific discourse. Definition combines the purposes of both real and nominal definition and is promoted by the Nyaya philosophers of India. Another important method of science and philosophy is induction. In a narrow sense induction is a method of generalization to all cases from the observation of particular cases. In a broad sense induction is a method for reasoning from some observed fact to a different fact not involved in the former. We understand induction in the broad sense though more often we shall actually be concerned with induction in the narrow sense. How can our limited experience of nature provide the rational basis for making knowlege claims about unobserved phenomena?