The relationship between yin and yang is often described in terms of sunlight playing over a mountain and in the valley. Yin (literally the 'shady place' or 'north slope') is the dark area occluded by the mountain's bulk, while yang (literally the 'sunny place' or 'south slope') is the brightly lit portion. As the sun moves across the sky, yin and yang gradually trade places with each other, revealing what was obscured and obscuring what was revealed.
Western society generally believes the Yin and Yang denote the positives and negatives in life, probably partly due to "western" religions having good and evil forces (Christianity for example), however it seems to me that this is talking about a middle path.
Taoism and Buddhism often preach the road of Moderation (the middle path) and that fits in well with the mountain metaphor in the sentence above.
As the sun moves across the sky, yin and yang gradually trade places with each other, revealing what was obscured and obscuring what was revealed.
To me, this seems to be saying that we should not rely on what side of the mountain is "lit", rather we should examine both sides. Thus we should do our best to learn about the world we live in, in all aspects.
Science and religion are on opposite sides of the mountain. If you focus solely on one side of the mountain, you will be blinding yourself to the other side.
How can you live a fulfilled life in ignorance?
"Religion and science are the two wings upon which man's intelligence can soar into the heights ... It is not possible to fly with one wing alone!" - `Abdu'l-Bahá
