"The advocates of the doctrine of action at a distance, however... say... Do we not see an instance of action at a distance in the case of a magnet... Besides this, Newton's law of gravitation... asserts not only that the heavenly bodies act ... across immense intervals of space... on one another with precisely the same force as if the strata beneath which each is buried [were] non-existant. If any medium takes part... it must surely make some difference whether the space... contains nothing but this medium, or whether it is occupied by the dense matter of the earth or of the sun."