Any correct explanation of the universe must postulate an intuitive knowledge of the existence of the external world, of self, and of God. The desire for scientific unity, however, has occasioned attempts to reduce these three factors to one, and according as one or another of the three has been regarded as the all-inclusive principle, the result has been Materialism, Materialistic Idealism, or Idealistic Pantheism. This scientific impulse is better satisfied by a system which we may designate as Ethical Monism. We may summarize the present chapter as follows: 1. _Materialism_: Universe = Atoms. Reply: Atoms can do nothing without force, and can be nothing (intelligible) without ideas. 2. _Materialistic Idealism_: Universe = Force + Ideas. Reply: Ideas belong to Mind, and Force can be exerted only by Will. 3. _Idealistic Pantheism_: Universe = Immanent and Impersonal Mind and Will. Reply: Spirit in man shows that the Infinite Spirit must be Transcendent and Personal Mind and Will. We are led from these three forms of error to a conclusion which we may denominate 4. _Ethical Monism_: Universe = Finite, partial, graded manifestation of the divine Life; Matter being God’s self-limitation under the law of necessity, Humanity being God’s self-limitation under the law of freedom, Incarnation and Atonement being God’s self-limitations under the law of grace. Metaphysical Monism, or the doctrine of one Substance, Principle, or Ground of Being, is consistent with Psychological Dualism, or the doctrine that the soul is personally distinct from matter on the one hand and from God on the other. I. Materialism. Materialism is that method of thought which gives priority to matter, rather than to mind, in its explanations of the universe. Upon this view, material atoms constitute the ultimate and fundamental reality of which all things, rational and irrational, are but combinations and phenomena. Force is regarded as a universal and inseparable property of matter. The element of truth in materialism is the reality of the external world. Its error is in regarding the external world as having original and independent existence, and in regarding mind as its product.