Natural science has been so busy with the investigation of the physical world that it has had time to remember but little besides objects in the external world.  And yet what are objects in the external world without a subject to know them?[16] And what are the hypotheses which science frames in order to explain phenomena but syntheses of factors framed in consciousness?[17] What are laws of Nature but mental constructions framed concerning similar ways of behaviour on the part of a large number of objects?  What are the fundamental conceptions which serve as the very groundwork of the whole of science but concepts which are explanations of phenomena and not themselves phenomena?[18]

Wherever we look, we find that our view [p.64] of Nature is in the first place a result as well as a conviction of the content of consciousness; that we do not perceive things and their qualities in a form of immediacy, but only after they have entered into consciousness are we able to know what external objects really are.  The constructions of science in the form of hypotheses and laws are a proof that the reality of the physical world and its meaning are known only in so far as they are known by mind, and in so far as the universal (which is a mental content) explains the particular (which may or may not be an object in the external world).

Eucken emphasises this truth in several of his books, and whenever the truth is borne in mind the scientist becomes aware of the existence of a reality beyond that of the objects of sense.  And even when the scientist is unaware of the mental qualities which operate in perceiving external objects and of the generalisations formed as the result of the impressions left by the objects in the mind, he uses these all the same.  Professor Haeckel (one of Professor Eucken’s colleagues in Jena) starts out in The Riddle of the Universe with the strong hope of reducing the whole universe (including God) into a state of material substance, and ends with a kind of peroration on the virtues of the new goddesses, the True, the Good, and the Beautiful.

[p.65] But an increasing number of scientists to-day are aware of the limits of science.  They know that the mental models which they have to frame in order to interpret phenomena are not material things, and exist nowhere except in a world of mind and meaning.  Eucken’s conclusion then is that what knows and interprets is a mental quality.  He would rather call it the life of the spirit of man, or the spiritual life.  A non-sensuous power has to operate in order that the physical world may be known at all; that power has, further, in a manner unknown, to gather the fragmentary impressions of the senses, turn them into that which is mental, combine them into what is termed meaning.