Lecture II
Now everything in the world depends like the pendulum upon periodicity. When the pendulum has swung up to a certain point it descends again and then rises up to the same height on the opposite side through the force it acquires in its descent. In the same way that the pendulum can go not only up to here but has to return to the level it reached before it descended are sleeping and waking opposed to one another. Roughly it may be expressed thus.
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