The quantity i cannot, of course, be a real number since the product of a real number with itself is always positive (or zero, if the number is itself zero). For this reason the term imaginary has been conventionally applied to numbers whose squares are negative. However, it is important to stress the fact that these 'imaginary' numbers are no less real than the 'real' numbers that we have become accustomed to. As I have emphasized earlier, the relationship between such 'real' numbers and physical reality is not as direct or compelling as it may at first seem to be, involving, as it does, a mathematical idealization of infinite refinement for which there is no clear a priori justification from Nature. (p. 87)
펜로즈의 말: 허수도 실수와 똑같은 정도로 실재한다. 실수도 어차피 인간이 만들어낸 것이고 자연이 실수로 기술된다는 어떠한 선험적 정당성도 없다.