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The terms "a priori" and "a posteriori" are used in philosophy to distinguish between the two different types of propositional knowledge. Thus, attempts to define clearly or explain a priori and a posteriori knowledge are part of a central thread in epistemology, the study of knowledge. Since the definitions and usage of the terms are disputed and have evolved in the history of philosophy, it is difficult to provide proper definitions of them. Rough and oversimplified explanations are as follows: a priori knowledge is independent of experience, while a posteriori knowledge is dependent on experience. Lawyers sometimes use "a priori" to describe a step in an argument the truth of which can be deduced entirely from the truth of the premises. "A posteriori", on the other hand, requires a bit more evidence.