It is a theorem in geometry
So, what is the Pythagorean theorem all about? Simply put, it is a theorem in geometry that states that in a rightangled triangle the area of the square on the longest side, the side opposite to the right angle, is equal to the sum of the areas of the squares of the other two sides.
At first glance, no rational reason is apparent that this should be the case. Of course we all know that the theorem is correct. It is taught in all the schools, isn't it. But how do we know this to be so, with absolute certainty? Pythagoras might have answered that it is possible for us to know this, because we are human beings that have the capability to see with the mind. When we do this, we also discover a bit more of what a human being is. And so, Pythagoras would say, "prove it to yourself that I am right," and thereby discover who you are as a human being.
