Abstract
IN TRYING TO ACHIEVE THE SIMPLICITY OF, IN SOME SENSE, scientific statements one must either, in the counsels of the wise, be very laboured or very brief. I want to be very brief and to use an essay as an essay, an attempt to try something out or on before a learned audience, or a timely speculation (that is, in time to be stopped) about whether a possible future voyage is worth undertaking, not an account of actual discovery.

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