I have already commented on the fact that today there is really general agreement upon the truths of mathematics alone, for these are the most trivial of all. No one can say that he finds mathematical truths through external experience; we find them through inwardly realizing them. If one wants to show that the three angles of a triangle make 18o°, then one draws a line through the apex which is parallel to the base and lays the three angles together fan-wise; then angle a = d, b = e, c = itself, and so the three angles are equal to a straight line, that is, 18o°. Anyone who has once grasped this knows that it must be so, once and for all, just as one knows that 3 x 3 = 9 after it has once been grasped.
To be continued
Source: Rudolf Steiner – GA 102 – The Influence of Spiritual Beings on Man: Lecture IX – Berlin, 1st June 1908