Knowledge

Knowledge (from Middle English knowlechen, to acknowledge) is "the fact or condition of knowing something with familiarity gained through experience or association" and "acquaintance with or understanding of a science, art, or technique." Another definition of note is "the circumstance or condition of apprehending truth or fact through reasoning." "Justified true belief" is the definition given by Plato. It is compared with synonyms "learning", "erudition," and "scholarship" as applying to "facts or ideas acquired by study, investigation, observation, or experience." On the other hand, the Catholic Encyclopedia notes, "Knowledge, being a primitive fact of consciousness, cannot, strictly speaking, be defined…"

Philosophy of knowledge
The study of knowledge is known as epistemology. As it pertains to knowledge, epistemology is concerned with the sources and limitations of knowledge. Epistemology is also the study of justification for beliefs. Evidentialism is a philosophy asserting that whether or not a belief is justified is dependent upon the evidence supporting it.

Common knowledge
Common knowledge is that which is known among members of a group to be mutually known by all members. Note that it is not sufficient to be merely mutually known, but it must be known to be mutually known. Common knowledge becomes significant in social interactions and, despite its rather obvious nature, was not articulated as a phenomenon until the 20th century. Secondhand or unverifiable knowledge is called hearsay. If it gets passed on long enough, it becomes an urban legend.

A priori knowledge
A priori knowledge refers to knowledge that is not justified by reference to empirical facts about the world as such. It often involves definitions and logical systems. For example, we know that a "married bachelor" is a logical impossibility because it is in conflict with the definition of bachelor. Mathematical knowledge may also be classified as a priori. The rationalist tradition of philosophy is associated with an emphasis on a priori knowledge. It is also often associated with excessive armchair theorizing.

A posteriori knowledge
A posteriori knowledge is the opposite of a priori knowledge. It involves facts about the empirical world, and so is associated with the philosophical tradition of empiricism. It may involve objective or subjective knowledge gained through either the unaided senses or with the aid of some kind of instrumentation.

Scientific knowledge
Scientific knowledge is that which is accepted by the scientific community as having been acquired through use of the scientific method. What counts as strictly "scientific knowledge" (as opposed to non-scientific knowledge and pseudoscience) is a matter of debate due to the problem of demarcation.

Situated knowledge
Situated knowledge is any knowledge that is only valid or relevant in a specific setting. Thus, the term "situated knowledge" is often associated with relativist or postmodernist philosophies.

Sources of knowledge
Sources are dictated by the philosophy used and the kind of knowledge desired. The Bible is not a good source of scientific knowledge, for example. Much scientific knowledge is, however, gained through experience. This includes perception, especially through experimentation and observation, memory of past experience, and, to acquire self-knowledge, introspection.

Another significant source of scientific knowledge is reason, which uses logic and, to some degree, intuition. Inductive reasoning, emphasized by modern science, starts with effects and infers causes. Deductive reasoning starts with assumptions and derives outcomes.

Sources of non-scientific knowledge include revelation, meditation, and esoteric or secret knowledge. According to one myth, knowledge can be gotten by eating an apple from the tree of knowledge of good and evil.