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. (I, 129)
To understand what exactly is meant by generalisation we can use a small example. When we say crows are black it is based on observations of many crows, their habit and behaviour which have led to generalisation. Same with any other common knowledge. The science of Raja Yoga proposes to give us such a means of observing the internal state. The instrument is mind itself. The power of attention, when properly guided and directed towards the internal world, will analyse the mind and illumine facts for us. This is our only means of knowledge.