我對幾何原本不確定，可能需研究一下這個

Euclid’s Postulate 2

“A terminated line can be further produced indefinitely.”

In simple words what we call a line segment was defined as a terminated line by Euclid. Therefore this postulate means that we can extend a terminated line or a line segment in either direction to form a line. In the figure given below, the line segment AB can be extended as shown to form a line.