It is not from without but from within that doubts arise as to realistic truth. They arise when the mind has acquired power over and above what is needed for bodily uses, and begins to think on its own account. Sentiments are felt which do not depend on or refer to bodily life, and a new intellect has to be formed to explain and protect these sentiments. This new intellect is Philosophy. It is the science and practical conduct of mind considered as abstracted from body. Much of the obscurity of philosophy is traceable to the superstition of a fixed standard of truth which must be recognised universally. We are reluctant to accept philosophical hints and inferences because they conflict with truths that have been physically verified. Or--which is more common--we take up a few philosophical propositions and tack on to them all the science we know, believing they make a homogeneous whole, because truth must be self-consistent. Time and labour would be spared if we could be told at the right moment that truth is expedience[4], and that there is no need to harmonise philosophy and science. We are each of us two men in one, and each of these men must be allowed to think for himself. There is no reason why they should quarrel; there is no reason why they should even argue. The science in our mind should not be ousted to make room for the philosophy; let them exist together and work alternately. When the mariner is at sea he must mind his ship and study the weather; when he is on shore he may neglect both. So when we are navigating the body we have to think in categories proper to its safety; as philosophers we dismiss the realistic categories and think in other forms, but we need not then call the realism false or foolish. In its proper place it is right and true[5]. Between realism and substantialism there is therefore no necessary conflict or competition. They are each indispensable. It is absurd to carry realism into philosophy, and no less absurd to carry substantialism into common affairs, or to reproach a substantialist because he acts and speaks occasionally like other people. It is probable however that in a community largely composed of substantialists the realism of common action would be less stringent than is now found necessary. [Footnote 3: _Will in Nature_, 'Physiology of Plants.'] [Footnote 4: This does not apply to truth in the sense of veracity.]