Euclid’s Elements, Book 1, Definition 4 of a straight line

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I’m very confused by Euclid’s definition 4 in Book 1 of a straight line

  1. A straight line is a line which lies evenly with the points on itself

I’ve read elsewhere that Euclid’s axioms aren’t as complete / formally defined as in other texts. But I’m curious if I’m simply reading this wrong.

By this definition, wouldn’t every line be a straight one? Do not all lines lie evenly with the points on themselves?