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24 it. Before consciousness can come, a certain degree of activity in the movement must be reached. This requisite degree is called the 'threshold;' but the height of the threshold varies under different circumstances: it may rise or fall. When it falls, as in states of great lucidity, we grow conscious of things of which we should be unconscious at other times; when it rises, as in drowsiness, consciousness sinks in amount. This rising and lowering of a psycho-physical threshold exactly conforms to our notion of a permanent obstruction to the transmission of consciousness, which obstruction may, in our brains, grow alternately greater or less.

The transmission-theory also puts itself in touch with a whole class of experiences that are with difficulty explained by the production-theory. I refer to those obscure and exceptional phenomena reported at all times throughout human history, which the 'psychical-researchers,' with