We see to-day the most violent conflict between Materialism and Spiritualism. This conflict simply rests on the fact that human beings simply will not see what deep foundations this utterance has: — The truth always lies midway between two directly opposite associations.
My dear friends, is it true that God is within us? Is it true then that we are in God? It is true that we are in God. These two assertions are direct opposites. Both are true. God is in us, and we are in God; but the two assertions are polar opposites. The real truth, the whole truth lies between the two. The nature of all the conflict of ideas in the world rests on this — that human beings always tend to a one-sidedness, which is true, but only a one-sided truth; whereas the real truth lies between two opposite assertions. We must know both in order to get at the reality.
For instance, to-day, in the present state of the evolution of the world, one must have the most earnest will to learn all we can of material existence above all, and not propagate the desire of those people who say: “We will only occupy ourselves with the Spirit: we do not want to know Matter.” To learn as much as possible of Matter is one side of human cognition, one thing for which the Will of man, must strive. On the other hand one must learn to know the Spirit, because between those two, lies what we are, and ought really to strive for. Both are wrong. — those who say the world is only Matter and those who say the world is only Spirit are wrong.
Source: Rudolf Steiner – GA 203 – Social life: Lecture 1 – Dornach, January 21, 1921