text:  Is the proposition that the angles of a triangle are equal to two right angles more certain than the ruin of a system under which such a state of things was tolerated? How then does it come to pass that the same people who cling to one set of truths reject the other with obstinate incredulity? Cicero shall account for it:--"Sensus nostros non parens non nutrix non poeta non scena depravat; animis omnes tendentur insidiae