# Definition:A Priori

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## Definition

**A priori** knowledge is the sort of knowledge which comes from reason alone.

That is, it does not require the exercise of experience to know it.

Thus the truth value of a statement can be decided by a logical argument whose premises are definitions.

## Examples

### Probability

The following is an example of * a priori* reasoning:

*The probability of rolling a $6$ on a fair die is $\dfrac 1 6$*.

The truth of the above statement is dependent upon:

- the definition of the term
**probability** - the definition of a
**fair die**.

### A Legal Statement

The following is an example of * a priori* reasoning:

*If Fred Bloggs has committed a crime, then he is guilty.*

The truth of the above statement is dependent upon the definition of the term **guilty**, which, in natural language simply means **having committed a crime**.

Contrast this with:

*Fred Bloggs has committed the crime of usury*

the truth value of which is dependent upon **knowing** whether or not he actually did it.

## Also see

## Linguistic Note

The phrase * a priori* derives from the Latin for

**from what comes before**.

## Sources

- 1998: David Nelson:
*The Penguin Dictionary of Mathematics*(2nd ed.) ... (previous) ... (next): Entry:**a priori** - 2008: David Nelson:
*The Penguin Dictionary of Mathematics*(4th ed.) ... (previous) ... (next): Entry:**a priori** - 2014: Christopher Clapham and James Nicholson:
*The Concise Oxford Dictionary of Mathematics*(5th ed.) ... (previous) ... (next): Entry:**a priori**