All things exist for us only in so far as they are perceptible to us. They exist as appearances, as the sum of their properties. If now the thinking mind would have anything made wholly manifest, wholly perceptible to sense--would seek to have something made wholly and entirely _appearance_, there always remains a residue that refuses to be made manifest, refuses to be made perceptible to sense. Speaking generally, one may say: Applied thought seems to conduct to a something lying at the foundation of things, to a constant in itself, of which all properties, all in things that is perceptible to sense, are only so many different expressions. The idea that all that exists does so in virtue of a constant in itself, presents itself as a _necessity of thought_, which science must oppose by every means if she would retain her title to the name of science. Since this constant in itself is of necessity an imperceptible to sense, it imposes no restrictions upon apprehension. One is perfectly at liberty to conceive of it in quite contrary forms--as matter or substance, equally as well as under the form of force. If one holds by the former mode of conceiving it, then, whatever the guise its elaboration in thought may assume, one belongs to the school _of materialism_. If, on the contrary, one holds by the latter mode of apprehension, one then belongs, quite independent of the form its detailed elaboration in thought may assume, to the _idealistic_ school. For the correct appraisement of our whole mental life, however, it is important clearly to understand that the opposition is only an apparent one. Both alike have one common root in the idea of an _unconditioned constant_ lying at the foundation of things, which, summed up, may be designated as _the substans_ (_das Substans_) of all appearances. _The substance_, accordingly, results purely as the material form of this _substans_, while the force represents its immaterial form: the one being as well--and as ill--authenticated as the other, since one knows nothing of either, nor ever can know anything.