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.
Philosophers take up the question of knowledge in Epistemology and Methodology.
C.S. Lewis said, "All our knowledge of the universe beyond our immediate experiences depends on inferences from these experiences." 
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.
The law of evidence also distinguishes between statements made of one's own knowledge and statements made on information and belief, which are statements of second-hand information that the person making the statements believes to be true.
Atheism and knowledge
See also: Atheism and knowledge and Atheism and epistemology
Epistemology is the analysis of the nature of knowledge, how we know, what we can and cannot know, and how we can know that there are things we know we cannot know. In other words, it is the academic term associated with study of how we conclude that certain things are true.
The Christian apologist Jason Petersen wrote about atheism and epistemology:
|“|| The atheist’s foundation for reasoning must be inductive. The reason being is because in the atheist worldview there is no ultimate starting point for knowledge. It has to be induced. If atheism is true, there can be no universal values - nature is all there is. But nature itself provides no benefit for knowledge. Nature itself is not knowledge. Thus, because there would be no ultimate universal standard for reasoning , there would be no starting point for knowledge. When humans came into existence in an atheist worldview, there wasn’t an embodiment of knowledge in which they could deduce the possibilities, rather, they had to build possible inferences while starting from nothing.
Most atheists agree that their foundation for reasoning is inductive. However, what a lot of people are unaware of is that inductive reasoning does NOT deal in absolutes. It only deals with what is possible or what is probable. This means that given that all of the premises in an inductive argument are true, the conclusion can still be false.
If an atheist agrees that their foundation for knowledge is inductive, then they must live consistently with that implication in order to have a self consistent foundation for knowledge. But if the foundation of knowledge is no more than induction, then it entails that one can not be ultimately certain about anything. If the atheists say that their foundation for knowledge is inductive, but yet they make absolute claims, then the atheist is contradicting their own epistemology. This is because the atheist, who uses inductive reasoning as their foundation for knowledge, has limited themselves to what they believe is probable, but they can’t make absolute claims due to the limitations of inductive reasoning.
- ↑ Atheism by Matt Slick
- Putting the Atheist on the Defensive by Kenneth R. Samples, Christian Research Institute Journal, Fall 1991, and Winter 1992, page 7.
- Atheists don’t own reason by Tom Gilson
- Why the Burden of Proof is on the Atheist by Professor Ralph McInerny
- Theism, Atheism, and Rationality by Alvin Plantinga
- ↑ "1", A Christian's Guide to Critical Thinking. Eugene, OR: Wipf and Stock Publishers, 336. ISBN 1-59752-661-4. Retrieved on 16.2.2012.
- ↑ The Epistemological Argument against Atheism