It will be found that the intuition works most readily in respect to those subjects which most habitually occupy our thought; and according to the physiological correspondences which we have been considering this might be accounted for on the physical plane by the formation of brain-channels specially adapted for the induction in the molecular system of vibrations corresponding to the particular class of ideas in question. But of course we must remember that the ideas themselves are not caused by the molecular changes but on the contrary are the cause of them; and it is in this translation of thought action into physical action that we are brought face to face with the eternal mystery of the descent of spirit into matter; and that though we may trace matter through successive degrees of refinement till it becomes what, in comparison with those denser modes that are most familiar, we might call a spiritual substance, yet at the end of it it is not the intelligent thinking principle itself. The criterion is in the word "vibrations." However delicately etheric the substance its movement commences by the vibration of its particles, and a vibration is a wave having a certain length, amplitude, and periodicity, that is to say, something which can exist only in terms of space and time; and as soon as we are dealing with anything capable of the conception of measurement we may be quite certain that we are not dealing with Spirit but only with one of its vehicles. Therefore although we may push our analysis of matter further and ever further back--and on this line there is a great deal of knowledge to be gained--we shall find that the point at which spiritual power or thought-force is translated into etheric or atomic vibration will always elude us. Therefore we must not attribute the origination of ideas to molecular displacement in the brain, though, by the reaction of the physical upon the mental which I have spoken of above, the formation of thought-channels in the grey matter of the brain may tend to facilitate the reception of certain ideas. Some people are actually conscious of the action of the upper portion of the brain during the influx of an intuition, the sensation being that of a sort of expansion in that brain area, which might be compared to the opening of a valve or door; but all attempts to induce the inflow of intuitive ideas by the physiological expedient of trying to open this valve by the exercise of the will should be discouraged as likely to prove injurious to the brain. I believe some Oriental systems advocate this method, but we may well trust the mind to regulate the action of its physical channels in a manner suitable to its own requirements, instead of trying to manipulate the mind by the unnatural forcing of its mechanical instrument. In all our studies on these lines we must remember that development is always by perfectly natural growth and is not brought about by unduly straining any portion of the system.