Watch Hal Abelson explain this on video, right at the start of the first 1986 SICP lecture: <a href="https:&#x2F;&#x2F;www.youtube.com&#x2F;watch?v=2Op3QLzMgSY" rel="nofollow">https:&#x2F;&#x2F;www.youtube.com&#x2F;watch?v=2Op3QLzMgSY</a>