First and foremost there is a question that must be answered, an old question asked already in the Gospels: _What is Truth?_ I think clear definitions of fundamental ideas are most necessary. How shall we contrive a working definition of the conception "Truth"? Perhaps an allegory may help us. Imagine a gigantic prism extending in front of the sun, so that its rays are broken up, but suppose man entirely ignorant of this fact. I exclude the invisible, chemical and ultra-violet rays. Men who live in a blue-lit region will say: "The sun sends forth blue light only." They are right and yet they are wrong: from their standpoint they are capable of perceiving only a _fragment_ of truth. And so too with the inhabitants of the red, yellow, and in-between regions. And they will all scourge and slay one another to force _their_ belief in _their_ fragment upon the others--till, grown wiser through travelling in each others' regions, they come to the harmonious agreement that the sun sends out light of varying colours. That comprehends more truth, but it is not yet _the_ Truth. Only when the giant lens shall have recombined the split-up rays, and when the invisible, chemical and heat rays have given proof of their own specific effects, will a view more in accordance with the facts be able to arise, and men will perceive that the sun emits white light which is split up by the prism into differing rays with different peculiarities, which rays can be recombined by the lens into one mass of white light. This example shows sufficiently well that the road to Truth leads through far-reaching and comparative observations, the results of which must be controlled by the help of freely chosen _experiments_, until well-grounded hypotheses and theories can be put forward; but these hypotheses and theories will fall to the ground as soon as a single new observation or experiment contradicts them.