The only straight way to ascertain the importance of phosphorus to thought would be to find whether more is excreted by the brain during mental activity than during rest. Unfortunately we cannot do this directly, but can only gauge the amount of PO_{5} in the urine, and this procedure has been adopted by a variety of observers, some of whom found the phosphates in the urine diminished, whilst others found them increased, by intellectual work. On the whole, it is impossible to trace any constant relation. In maniacal excitement less phosphorus than usual seems to be excreted. More is excreted during sleep. The fact that phosphorus-preparations may do good in nervous exhaustion proves nothing as to the part played by phosphorus in mental activity. Like iron, arsenic, and other remedies it is a stimulant or tonic, of whose intimate workings in the system we know absolutely nothing, and which moreover does good in an extremely small number of the cases in which it is prescribed. The phosphorus-philosophers have often compared thought to a secretion. "The brain secretes thought, as the kidneys secrete urine, or as the liver secretes bile," are phrases which one sometimes hears. The lame analogy need hardly be pointed out. The materials which the brain _pours into the blood_ (cholesterin, creatin, xanthin, or whatever they may be) are the analogues of the urine and the bile, being in fact real material excreta. As far as these matters go, the brain is a ductless gland. But we know of nothing connected with liver-and kidney-activity which can be in the remotest degree compared with the stream of thought that accompanies the brain's material secretions. CHAPTER X. HABIT. =Its Importance for Psychology.=--There remains a condition of general neural activity so important as to deserve a chapter by itself--I refer to the aptitude of the nerve-centres, especially of the hemispheres, for acquiring habits. _An acquired habit, from the physiological point of view, is nothing but a new pathway of discharge formed in the brain, by which certain incoming currents ever after tend to escape._ That is the thesis of this chapter; and we shall see in the later and more psychological chapters that such functions as the association of ideas, perception, memory, reasoning, the education of the will, etc., etc., can best be understood as results of the formation _de novo_ of just such pathways of discharge.