Everywhere in life we find duality: light and shade, positive and negative, male and female, left and right, right and wrong, good and evil. Duality is deeply rooted in the nature of all existence, and he who wants to understand nature, must be aware of this duality and always have it clearly in mind. Only when we perceive this duality in our own life, can we come to understand the world. The pupil must make it his duty to think in terms of these dualities. He should never have one aspect only in mind but he should always be aware of both sides when thinking. […] Only when one imposes on oneself the inner duty never to think in terms of a single aspect only, but to think always in terms of duality, can one come to know the full truth. When humanity learns to think in dualities, thinking will be factually correct and in accordance with reality.
Source (German): Rudolf Steiner – GA 89 – Bewusstsein – Leben – Form – Berlin, April 3, 1905 (page 291)
Translated by Nesta Carsten-Krüger