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I regret that it has been necessary for me in this lecture to administer such a large dose of four-dimensional geometry. I do not apologise, because I am really not responsible for the fact that nature in its most fundamental aspect is four-dimensional. Things are what they are; and it is useless to disguise the fact that ‘what things are’ is often very difficult for our intellects to follow. It is a mere evasion of the ultimate problems to shirk such obstacles.
