In these words Liebig expresses the prevalent views which have accustomed themselves to look upon material and spiritual differences as absolute opposites. But the untenability of such a distinction is vaguely felt even by the just quoted advocate of this view, who speaks of material interests and of a mental penetration which is the condition for our existence, welfare, and development. But what else does the term material interests mean but the abstract expression of our existence, welfare, and development? Are not these the concrete content of our material interests? Does he not say explicitly that the penetration of the wonders of creation promotes our material interests? And on the other hand, does not the promotion of our material interests require a penetration on our part of the wonders of creation? In what respect are our material interests different from our mental penetration of things? The superior, spiritual, ideal, which Liebig in conformity with the views of the world of naturalists opposes to our material interests, is only a special part of those interests. Mental penetration and material interests differ no more than the circle differs from the square. Circles and squares are contrasts, but at the same time they are but different and special classes of form in general. It has been the custom, especially since the advent of Christian times, to speak contemptuously of material, perceptible, fleshly things which are destroyed by rust and moths. And nowadays people continue on this conservative track, although their antipathy against perceptible reality has long disappeared from their minds and actions. The Christian separation of mind and body has been practically abandoned in the age of natural science. But the theoretical solution of the contradiction, the demonstration that the spiritual is material and the material at the same time spiritual, by which the material interests would be freed from the stigma of inferiority, has not yet been forthcoming.