Euclid tried to keep his list of postulates shorter than what you saw on the previous page, but it was later discovered that he made some additional assumptions, so more postulates were added to clarify the proofs.  However, not everything that seems basic or "obvious" need be classified as a postulate, and mathematicians since Euclid try to keep the list as short as possible.  For example, the following fact seems about as obvious and simple as any of the above postulates:

If two lines intersect, then they intersect in exactly one point.

But this in fact is a Theorem since it follows logically from postulate 1, and we can prove it using the contrapositive of that postulate provided we first translate the postulate to if-then form:

Postulate 1: If A and B are two different points, then there is exactly one line containing them.

Contrapositive: If there are no lines or more than one line containing points A and B, then A and B are the same point.

Thus, if points A and B are contained in both lines l and m, then they must be the same point. 

Here is another theorem that seems as simple and obvious as a postulate:

There is exactly one plane containing a given line and point not on the line.

It can be proved directly using postulates 2 and 7:

Proof: By postulate 2, there are 2 points on the given line.  Since the given point is not on that line, the given point is a 3^rd point, and these three points are noncollinear.  By postulate 7, these three points lie in exactly one plane. 

These are not all the postulates of Euclidean geometry.  Another postulate that Euclid gave (though in a slightly different form) is the so-called Parallel Postulate:

9. Given a line and a point not on that line, there is exactly one line containing the given point and parallel to the given line.

Euclid did not like this postulate because it seemed more complicated than the others, and for more than 1000 years people tried to prove it on the basis of the other postulates.  Eventually it was understood that this postulate cannot be proved from the others, and it is possible to assume it false and derive a different kind of geometry (called "non-Euclidean").  In fact, the geometry Einstein used to describe gravitational forces in the universe is a form of non-Euclidean geometry.  According to Einstein's Theory of Relativity, all lines in space eventually intersect, so there is no such thing as true parallel lines.  But for all practical purposes, Euclidean geometry does work for most of the things we do (as in making buildings) and it is far simpler to use than Einstein's geometry.  It is also more intuitive and easier to understand.

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