The words "analysis" and "synthesis" are among the most widely used and misused terms in the history of philosophy. They were originally used in geometrical reasoning during the age of Euclid to describe two opposing, but complementary, methods of arguing (roughly equivalent to deduction and induction). Since then philosophers have used them not only in this way, but also to refer to distinctions of various sorts between types of judgment or classes of propositions. To some they are regarded as defining differences of kind, while others regard them as defining differences of degree. Moreover, they have been connected in numerous different ways with other distinctions, such as "a priori vs. a posteriori" or "necessary vs. contingent". Some philosophers have become so frustrated at the ambiguity attached to the various uses of the terms "analytic" and "synthetic" that they have given up all hope of assigning a coherent meaning to this distinction.