Epistemology
Types

Epistemologists distinguish between different types of knowledge. Their primary interest is in knowledge of facts, called propositional knowledge. It is a theoretical knowledge that can be expressed in declarative sentences using a that-clause, like "Ravi knows that kangaroos hop". For this reason, it is also called knowledge-that. Epistemologists often understand it as a relation between a knower and a known proposition, in the case above between the person Ravi and the proposition "kangaroos hop". It is use-independent since it is not tied to one specific purpose. It is a mental representation that relies on concepts and ideas to depict reality. Because of its theoretical nature, it is often held that only relatively sophisticated creatures, such as humans, possess propositional knowledge.

Propositional knowledge contrasts with non-propositional knowledge in the form of knowledge-how and knowledge by acquaintance. Knowledge-how is a practical ability or skill, like knowing how to read or how to prepare lasagna. It is usually tied to a specific goal and not mastered in the abstract without concrete practice. To know something by acquaintance means to be familiar with it as a result of experiental contact. Examples are knowing the city of Perth, knowing the taste of tsampa, and knowing Marta Vieira da Silva personally.

Another influential distinction is between a posteriori and a priori knowledge. A posteriori knowledge is knowledge of empirical facts based on sensory experience, like seeing that the sun is shining and smelling that a piece of meat has gone bad. Knowledge belonging to the empirical science and knowledge of everyday affairs belongs to a posteriori knowledge. A priori knowledge is knowledge of non-empirical facts and does not depend on evidence from sensory experience. It belongs to fields such as mathematics and logic, like knowing that 2 + 2 = 4 {\displaystyle 2+2=4}. The contrast between a posteriori and a priori knowledge plays a central role in the debate between empiricists and rationalists on whether all knowledge depends on sensory experience.

A closely related contrast is between analytic and synthetic truths. A sentence is analytically true if its truth depends only on the meaning of the words it uses. For instance, the sentence "all bachelors are unmarried" is analytically true because the word "bachelor" already includes the meaning "unmarried". A sentence is synthetically true if its truth depends on additional facts. For example, the sentence "snow is white" is synthetically true because its truth depends on the color of snow in addition to the meanings of the words snow and white. A priori knowledge is primarily associated with analytic sentences while a posteriori knowledge is primarily associated with synthetic sentences. However, it is controversial whether this is true for all cases. Some philosophers, such as Willard Van Orman Quine, reject the distinction, saying that there are no analytic truths.
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