Knowledge is distinct from information. Both knowledge and information consist of true statements, but knowledge is information that has a purpose or use. Philosophers would describe this as information associated with intentionality. The study of knowledge is called epistemology.
A common definition of knowledge is that it consists of justifiedtruebelief. This definition derives from Plato's Theaetetus. It is considered to set out necessary, but not sufficient, conditions for some statement to count as knowledge.
What constitutes knowledge, certainty and truth are controversial issues. These issues are debated by philosophers, social scientists, and historians. Ludwig Wittgenstein wrote "On Certainty" - aphorisms on these concepts - exploring relationships between knowledge and certainty. A thread of his concern has become an entire field, the philosophy of action.
In Jewish, Christian and Islamic traditions, there has always been a considerable tension on the issue of authority versus experience in the formation of knowledge. Early Christian philosophy contrasted revelation from God with knowledge gained by reason. St. Augustine for instance put the knowledge of classical philosophers, especially Plato, into a Christian framework. Experimental knowledge was discounted. Early Muslim philosophy, especially the Mutazilite school, medieval Jewish philosophy, and later Christian work, especially that of Thomas Aquinas, focused on Aristotle's views. These were vast controversies stretching over centuries. The (eventually dominant) Asharite school of Islamic scholars, for instance, strongly rejected most views of Aristotle, while the Roman Catholic tradition generally embraced them. Such efforts to provide an ethical or spiritual basis for the foundations of knowledge continue to this day in the sociology of knowledge, Islamization of knowledge, and the many and varied strains of economics.
A second way to derive knowledge is by observation and experiment. It is not free of uncertainty, as errors of observation or interpretation may occur, and any sense can be deceived by illusions.
Knowledge may also be derived by reason from either traditional, authoritative, or experiential sources or a combination of them. Inferential knowledge is based on reasoning from facts or from other inferential knowledge such as a theory.
However, if Fred acquired this propositional knowledge from an encyclopedia, he will not have acquired the skill of swimming: he has some propositional knowledge, but does not have any know-how. In general, one can demonstrate know-how by performing the task in question, but it is harder to demonstrate propositional knowledge.
Inferential knowledge is based on reasoning from facts or from other inferential knowledge such as a theory. Such knowledge may or may not be verifiable by observation or testing. For example, all knowledge of the atom is inferential knowledge. The distinction between factual knowledge and inferential knowledge has been explored by the discipline of general semantics.
Another problem with defining knowledge is known as the "Gettier problem". The Gettier problem arises when we give certain kinds of counterexamples to the JTB (justified true belief) definition. A counterexample is a case where the definition applies, but the word defined doesn't; or a case where the word defined applies, but the definition doesn't. Gettier counterexamples are examples where the definition, justified, true belief applies; but one nevertheless still doesn't have knowledge, so the word "knowledge" doesn't apply in that case.
