Complete knowledge of the nature of an analytic function must also include insight into its behavior for imaginary values of the arguments. Often the latter is indispensable even for a proper appreciation of the behavior of the function for real arguments. It is therefore essential that the original determination of the function concept be broadened to a domain of magnitudes which includes both the real and the imaginary quantities, on an equal footing, under the single designation complex numbers.
By then I knew that everything good and bad left an emptiness when it stopped. But if it was bad, the emptiness filled up by itself. If it was good you could only fill it by finding something better.