Belief

Belief or believing, in simplest terms, is the acceptance by the mind of something as true.  A belief is something which is believed. One can believe something to be true without having knowledge that it is true. Also, one can believe something to varying degrees of certainty (see also Probability).

Calling someone a believer often connotes belief in a god, especially the Christian God (in the U.S., anyway). There are many reasons people give for their belief in their god/s. Some amount to, “When I go outside and I look around I can only think god did it all.” Sometimes, though, the belief is related to some other supernatural or unsupported claim, such as ESP or alien abduction &mdash; or even to a philosophical position or political opinion.

Degrees of belief
Common phrases describing one's own degree of belief:
 * I know that
 * I believe that
 * I think that
 * I feel that
 * I [would] guess that
 * I [would] bet/wager that
 * I [would] like to think/believe that
 * I have no reason to doubt that
 * I have no reason not to believe/think that

Common phrases describing the degree of believability of a statement of fact:
 * It is true
 * It is certainly true
 * It is surely true
 * It is almost certainly/surely true
 * It is most likely true
 * It is likely true
 * It is probably true
 * It is possibly true
 * It is perhaps true
 * It may be true
 * It might be true
 * It should be true
 * It can/could be true

In addition, most of the above statements could be negated in two different ways, giving rise to different connotations. For example, "It is not likely true" is different from "It is likely not true".

Disbelief
The word disbelief has two possible connotations:
 * 1) lack of, or absence of, belief
 * 2) belief to the contrary

The two connotations are at the heart of the confusion between weak atheism (lack of belief in any gods) and strong atheism (positive belief that there are no gods).

See also Atheist vs. agnostic and Absolute certainty.

Belief vs. faith
Faith is sometimes defined as belief without evidence, or in the face of contradictory evidence. Clearly, "belief" does not always imply "faith," as one can believe something based on evidence or for other rational reasons (e.g., logical arguments). For example, one does not need to have faith that the sun will rise tomorrow, since the fact that the sun rises every day is supported by a lifetime of personal experience, thousands of years worth of recorded observations, and a scientific model of the earth's rotation on an axis.

Belief in
However, the definition of "faith" given above is usually not what religious people mean when they use the word: to them, having faith is closer to "trusting" than simply "believing" (see the Faith article for more information). The phrase "belief in" is sometimes used to imply this element of trust. For example, when Christians say they "believe in" God, they usually don't simply mean that they believe he exists, but that they believe he loves them, has a plan for their lives, will see them through hard times, etc. For this reason, when Christians ask what atheists "believe in", or whether an atheist "believes in" evolution or the big bang, the best response is likely to include an explanation of the difference between belief and faith.

Philosophically

 * How do we define belief and believing? If
 * A: Belief is a form of reason and reason denotes structure, then belief equals our awareness of the world through a structured combination of sensory input.
 * AA: Then the structure with the highest amount of order and inner coherence would be the most true. All closed logical speculations have a higher amount of truth then open and random speculations.
 * AB: If in theory no human mind can exist inside a closed logical mode, then all logical speculations must be inherently open, and open to random speculations. And the validity of these random speculations would only be laid down by the structured and organised mind mimicking a closed logical speculation system (dogma), then believe is always dogma and freedom is inclusion of all forms even if they contradict each other.
 * B: Belief is a lack of reason and would not denote structure but rather randomness projected on a structured consciousness.
 * BB: This would define all believe as raw sensory input and lead back to point A.