Hi, Please refer to the Pythagorean proof of the theorem that the angels in a triangle add to 180 degrees. The following link has the proof. http://www.cut-the-knot.org/triangle/pythpar/AnglesInTriangle.shtml You will note that this proof is based on the assumptions that angles on a straight line add to 180 degrees. But we have no proof of that fact. I mean we can measure it with a protractor, but is there a way to prove that? So I am wondering about the whole process of proving a theorem. We use some assumptions to prove a statement. The assumptions themselves are theorems that have to proved somewhere else. Is that the right way to think about it? Thanks.