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Without the definition of a straight line, how did your light and line segments come out of thin air? !
There is no geometric feature of a straight line. Why should we define a line segment connecting two points and distinguish it from a curve segment connecting two points? !
So your thinking direction has been reversed from the beginning.
It is precisely because there is a straight line with two points and only one straight line that we define what line segments and rays are, and then we find examples of line segments and rays in reality (application level). Mathematics is a tool, not a science, so it is not the other way around!
As for the first person to define a straight line? Learn about the five postulates (330 ~ 275 BC) in Euclid's Elements of Geometry!
Before that, there must have been the name straight line. After all, the pyramids can't be built without the help of line segments or light. However, no scientific system can describe them. So it's not a straight line here. He invented it.