Changes

==More questions==

==More questions==

More fundamentally, what does it mean for a point to be "given"? No comment. But it turns out that the three assumed constructions are all that we need to know about given points and line segments. We don't have to know how to give one or the other at the beginning of a problem, and we don't have to know how to do the three constructions. It is sufficient, in a proof, to just say, "Do it." What matters is that all of the constructions and proofs in Euclid's elements are built out of just those three constructions, and that we can take intersections of previously constructed lines and circles as given for doing further constructions.

More fundamentally, what does it mean for a point to be "given"? No comment from Euclid. But it turns out that the three assumed constructions are all that we need to know about given points and line segments. We don't have to know how to give one or the other at the beginning of a problem, and we don't have to know how to do the three constructions. It is sufficient, in a proof, to just say, "Do it." What matters is that all of the constructions and proofs in Euclid's elements are built out of just those three constructions, and that we can take intersections of previously constructed lines and circles as given for doing further constructions.

This is often caller "ruler and compass" construction, but it is important to understand that the ruler is unmarked, and the compass maintains a radius only as long as the center is unchanged. So to begin with, we cannot copy lengths from one place to another. Euclid addresses that lack as quickly as possible, in Proposition 2 of Book 1.

This is often caller "ruler and compass" construction, but it is important to understand that the ruler is unmarked, and the compass maintains a radius only as long as the center is unchanged. So to begin with, we cannot copy lengths from one place to another. Euclid addresses that lack as quickly as possible, in Proposition 2 of Book 1, as we have just seen.

The first and third of the Euclidean constructions are quite definite, but "as far as needed" is rather vague. Usually it means "until the line intersects some line or circle of interest". This commonly means that we have to prove that they will intersect before we can invoke this construction, or that the fact of intersection is something previously assumed.

The first and third of the Euclidean constructions are quite definite, but "as far as needed" is rather vague. Usually it means "until the line intersects some line or circle of interest". This commonly means that we have to prove that they will intersect before we can invoke this construction, or that the fact of intersection is something previously assumed.