Haeckel's own statement of this aspect of his philosophy is somewhat interesting. He says: "The opponents of the doctrine of evolution are very fond of branding the monistic philosophy grounded upon it as 'materialism' by comparing _philosophical_ materialism with the wholly different and censurable _moral_ materialism. Strictly, however, our 'monism' might as accurately or as inaccurately be called spiritualism as materialism. The real materialistic philosophy asserts that the phenomena of vital motion, like all other phenomena of motion, are effects or products of matter. The other opposite extreme, spiritualistic philosophy, asserts, on the contrary, that matter is the product of motive force, and that all material forms are produced by free forces entirely independent of the matter itself. Thus, according to the materialistic conception of the universe, matter precedes motion or active force; according to the spiritualistic conception of the universe, on the contrary, active force or motion precedes matter. Both views are dualistic, and we hold them both to be equally false. A contrast to both is presented in the _monistic_ philosophy, which can as little believe in force without matter as in matter without force."
It is evident that if Haeckel limits himself and his opponents to matter and force as the sole possible explanations of the universe, he may truly say that matter is inconceivable without force and force inconceivable without matter. But the question arises, What is the monistic power beyond these--the "power behind nature"? and as to the true nature of this the Jena philosopher gives us only vague generalities, though it is quite plain that he cannot admit a Spiritual Creator. Further, as to the absence of any spiritual element from the nature of man, he does not leave us in doubt as to what he means; for immediately after the above paragraph he informs us that "the 'spirit' and the 'mind' of man are but forces which are inseparably connected with the material substance of our bodies. Just as the motive-power of our flesh is involved in the muscular form-element, so is the thinking force of our spirit involved in the form-element of the brain." In a note appended to the passage, he says that monism "conceives nature as one whole, and nowhere recognizes any but mechanical causes." These assumptions as to man and nature pervade the whole book, and of course greatly simplify the task of the writer, as he does not require to account for the primary origin of nature, or for anything in man except his physical frame; and even this he can regard as a thing altogether mechanical.