If A makes a claim, and B then casts doubt on it, A's next move would normally be to provide justification. Empiricism (the evidence of the senses), authoritative testimony (the appeal to criteria and authority), and logical deduction are often involved in justification. Justification based theories of knowledge can be divided into irrationalism, which appeals to irrational criteria and authorities (feelings, faith) and panrationalism, which appeals to rational criteria and authorities (observation, intellectual intuition).
In some way, each of us is responsible for what we believe. Beliefs are not typically formed completely at random, and thus we have an intellectual responsibility, or obligation, to try to believe what is true and to avoid believing what is false. An intellectually responsible act is within one's intellectual rights in believing something; preforming it, one is justified in one's belief.
Thus, justification is a normative notion. The standard definition is that a concept is normative if it is a concept regarding or depending on the norms, or obligations and permissions (very broadly construed), involved in human conduct. It is generally accepted that the concept of justification is normative, because it is defined as a concept regarding the norms of belief.
The main theories of justification include:
Minority viewpoints include:
Not all justifiers have to be what can properly be called "evidence"; there may be some substantially different kinds of justifiers available to us. Regardless, to be justified, a belief has to have a justifier.
But this raises an important question: what sort of thing can be a justifier?
Three things that have been suggested are:
At least sometimes, the justifier of a belief is another belief. When, to return to the earlier example, the woman believes that her husband is having an affair, she bases that belief on other beliefs—namely, beliefs about the lipstick and perfume. Strictly speaking, her belief isn't based on the evidence itself—after all, what if she did not believe it? What if she thought that all of that evidence were just a hoax? What if her husband commonly wears perfume and lipstick on business trips? For that matter, what if the evidence existed, but she did not know about it? Then, of course, her belief that her husband is having an affair wouldn't be based on that evidence, because she did not know it was there at all; or, if she thought that the evidence were a hoax, then surely her belief couldn't be based on that evidence.
Consider a belief P. Either P is justified or P is not justified. If P is justified, then another belief Q may be justified by P. If P is not justified, then P cannot be a justifier for any other belief: neither for Q, nor for Q's negation.
For example, suppose someone might believe that there is intelligent life on Mars, and base this belief on a further belief, that there is a feature on the surface of Mars that looks like a face, and that this face could only have been made by intelligent life. So the justifying belief is: that face-like feature on Mars could only have been made by intelligent life. And the justified belief is: there is intelligent life on Mars.
But suppose further that the justifying belief is itself unjustified. It would in no way be one's intellectual right to suppose that this face-like feature on Mars could have only been made by intelligent life; that view would be irresponsible, intellectually-speaking. Such a belief would be unjustified. It has a justifier, but the justifier is itself not justified. In fact, more recent observations have shown that the "helmeted face" does not look the same up close, nor when viewed from the side.
In justificationism, criticism consists of trying to show that a claim cannot be reduced to the authority or criteria that it appeals to. That is, it regards the justification of a claim is primary, while the claim itself is secondary. By contrast, nonjustificational criticism works towards attacking claims themselves.
Bartley also refers to a third position, which he calls critical rationalism in a more specific sense, claimed to have been Popper's view in his Open Society. It has given up justification, but not yet adopted nonjustificational criticism. Instead of appealing to criteria and authorities, it attempts to describe and explicate them.
Philosophy professor and author Robert J. Fogelin claims to detect a suspicious resemblance between the Theories of Justification and Agrippa's five modes leading to the suspension of belief. He concludes that the modern proponents have made no significant progress in responding to the ancient modes of pyrrhonic skepticism.