Knowledge is the sum of what is known. There are said to be various kinds of knowledge: knowledge of matters of contingent fact (empirical knowledge), knowledge of necessary truths (a priori knowledge), and knowledge of matters of gods and religions (Divine revelation). This can mean that what constitutes 'knowledge', despite the concept involving incontrovertibility, is in fact contested around the world, between religions, among historians, scientists etc.
Many believe that sharing of knowledge is the best way to increase knowledge. Others believe so firmly in one kind of knowledge that they fear learning. Encyclopedias often contain a wealth of 'knowledge' unless they espouse only one single type of knowledge in which case they seem to become repositories of 'opinion'.
Knowledge is opposed to mere subjective opinion, or the body of facts, truths, or principles accumulated in the course of time.
General erudition or Polymath deals with knowledge of many things.
In law, knowledge can have various meanings:
- Actual knowledge is what a person actually knew at a relevant time.
- Constructive knowledge is what a person knew or should have known at a relevant time, i.e., what he would have known if he had exercised reasonable care.
- Imputed knowledge can be one person's knowledge imputed to another because of their legal relationship (e.g., agent and principal), knowledge that people are required to have ("Ignorance of the law is no excuse"), or knowledge that someone lacks because of willful ignorance.