The number of n element permutations is the product of the integers from 1 to n, which is called n factorial, and is written n!. Select the first (leftmost) book in n different ways, then the next book in n-1 ways, and so on until there is but 1 way to choose the last book. As described in the introduction, the number of outcomes is n×(n-1)×…×2×1 = n!.
Note that the n events are numerically independent, not physically independent. After selecting David Copperfield, we cannot select it again. yet after selecting any of the n books, we always have n-1 remaining books to choose from, then n-2 books, and so on. We only need numerical independence to multiply outcomes together.
For convenience, define 0! = 1.
(Limerick)
