The following outline is provided as an overview of and topical guide to knowledge: Knowledge is an awareness of facts (descriptive knowledge), a familiarity with individuals and situations (knowledge by acquaintance), or a practical skill (procedural knowledge). Descriptive knowledge, also called propositional knowledge, is often characterized as true belief that is distinct from opinion or guesswork by virtue of justification. While there is wide agreement among philosophers that propositional knowledge is a form of true belief, many controversies focus on justification. This includes questions like how to understand justification, whether it is needed at all, and whether something else besides it is needed. These controversies intensified in the latter half of the 20th century due to a series of thought experiments called Gettier cases that provoked alternative definitions. Knowledge can be produced in many ways. The main source of empirical knowledge is perception, which involves the usage of the senses to learn about the external world. Introspection allows people to learn about their internal mental states and processes. Other sources of knowledge include memory, rational intuition, inference, and testimony. According to foundationalism, some of these sources are basic in that they can justify beliefs, without depending on other mental states. Coherentists reject this claim and contend that a sufficient degree of coherence among all the mental states of the believer is necessary for knowledge. According to infinitism, an infinite chain of beliefs is needed.
Types of knowledge
By form A priori and a posteriori knowledge – these terms are used with respect to reasoning (epistemology) to distinguish necessary conclusions from first premises. A priori knowledge or justification – knowledge that is independent of experience, as with mathematics, tautologies ("All bachelors are unmarried"), and deduction from pure reason (e.g., ontological proofs). A posteriori knowledge or justification – knowledge dependent on experience or empirical evidence, as with most aspects of science and personal knowledge. Descriptive knowledge – also called declarative knowledge or propositional knowledge, it is the type of knowledge that is, by its very nature, expressed in declarative sentences or indicative propositions (e.g., "Capybaras are rodents", or "It is raining"). This is distinguished from what is commonly known as "know-how" or procedural knowledge (the knowledge of how, and especially how best, to perform some task), and "knowing of", or knowledge by acquaintance (the knowledge of something's existence). Experience – knowledge or mastery of an event or subject gained through involvement in or exposure to it. Empirical evidence – also referred to as empirical data, empirical knowledge, and sense experience, it is a collective term for the knowledge or source of knowledge acquired by means of the senses, particularly by observation and experimentation. After Immanuel Kant, it is common in philosophy to call the knowledge thus gained a posteriori knowledge. This is contrasted with a priori knowledge, the knowledge accessible from pure reason alone. Experiential knowledge – knowledge gained through experience. Explicit knowledge – knowledge that can be readily articulated, codified, accessed and verbalized. It can be easily transmitted to others. Most forms of explicit knowledge can be stored in certain media. The information contained in encyclopedias and textbooks are good examples of explicit knowledge. Extelligence – term coined by Ian Stewart and Jack Cohen in their 1997 book Figments of Reality. They define it as the cultural capital that is available to us in the form of external media (e.g., tribal legends, folklore, nursery rhymes, books, videotapes, CD-ROMs, etc.). Knowledge by acquaintance – according to Bertrand Russell, knowledge by acquaintance is obtained through a direct causal (experience-based) interaction between a person and the object that person is perceiving. Sense-data from that object are the only things that people can ever become acquainted with; they can never truly KNOW the physical object itself. The distinction between "knowledge by acquaintance" and "knowledge by description" was promoted by Russell (notably in his 1905 paper On Denoting). Russell was extremely critical of the equivocal nature of the word "know", and believed that the equivocation arose from a failure to distinguish between the two fundamentally different types of knowledge. Libre knowledge – knowledge released in such a way that users are free to read, listen to, watch, or otherwise experience it; to learn from or with it; to copy, adapt and use it for any purpose; and to share the work (unchanged or modified). Whilst shared tacit knowledge is regarded as implicitly libre, (explicit) libre knowledge is defined as a generalisation of the libre software definition. Procedural knowledge – also known as imperative knowledge, it is the knowledge exercised in the performance of some task. Commonly referred to as "knowing-how" and opposed to "knowing-that" (descriptive knowledge). Tacit knowledge – kind of knowledge that is difficult to transfer to another person by means of writing it down or verbalizing it. For example, that London is in the United Kingdom is a piece of explicit knowledge that can be written down, transmitted, and understood by a recipient. However, the ability to speak a language, knead dough, play a musical instrument or design and use complex equipment requires all sorts of knowledge that is not always known explicitly, even by expert practitioners, and which is difficult or impossible to explicitly transfer to other users. Threshold knowledge
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