In acquiring knowledge we make use of generalisations, and generalisation is based upon observation. We first observe facts, then generalise, and then draw conclusions or principles. The knowledge of the mind, of the internal nature of man, of thought, can never be had until we have first the power of observing the facts that are going on within. It is comparatively easy to observe facts in the external world, for many instruments have been invented for the purpose, but in the internal world we have no instrument to help us. Yet we know we must observe in order to have a real science. Without a proper analysis, any science will be hopeless — mere theorising. And that is why all the psychologists have been quarrelling among themselves since the beginning of time, except those few who found out the means of observation.