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The reason that we can't understand these motions; the brain if you like can't understand these motions, is that there's continual conflict between what is upright and whether you should lean to balance yourself in the environment or whether you're actually experiencing a sideways acceleration force. You imagine being on a bicycle or a motorbike; you go round a corner, you lean into the corner which remains perfectly upright in physics. You don't so that in a car, you don't do that on a ship - you're actually struggling to find out what is upright and what's the best way of dealing with it.