# Axiom facts for kids

An **axiom** is a concept in logic. It is a statement which is accepted without question, and which has no proof. The axiom is used as the premise or starting point for further reasoning or arguments, usually in logic or in mathematics.

This means it cannot be proved within the discussion of a problem. So inside some discussion, it is thought to be true. There are many reasons why it has no proof. For example,

- The statement might be obvious. This means most people think it is clearly true. An example of an obvious axiom is the principle of contradiction. It says that a statement and its opposite cannot both be true at the same time and place.
- The statement is based on physical laws and can easily be observed. An example is Newton's laws of motion. They are easily observed in the physical world.
- The statement is a proposition. Here, an axiom is any mathematical statement that serves as a starting point from which other statements are logically derived. This means we care more about what
*happens*if the axiom is true. Whether the proposition is empirically true is not the business of logic. This is a more modern definition of an axiom.

Logic can be used to find theorems from the axioms. Then those theorems can be used to make more theorems. This is often how math works. Axioms are important because logical arguments start with them.

## Euclid's axioms

Euclid of Alexandria was a Greek mathematician. Around the year 300 BC, he made a list of axioms:

- Two numbers that are both the same as a third number are the same number.
- If A and B are two numbers that are the same, and C and D are also the same, A+C is the same as B+D.
- If A and B are two numbers that are the same, and C and D are also the same, A-C is the same as B-D.
- Two shapes that fill exactly the same space are the same shape.
- If you divide a number by anything more than 1, the quotient (result) will be less than the original number.

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