A Latin expression, used in English to mean an essential or necessary quality or condition - that without which something would not be what it is.
The Dragon's use of the term emphasizes his philosophical (or mock- philosophical) bent. He is criticizing the human tendency to acknowledge only isolated facts. "There are no such facts," he says. And in this Gardner makes him echo a famous thought from Friedrich Nietzsche that "facts are precisely what there is not, only interpretations" (quoted in The Johns Hopkins Guide to Literary Theory and Criticism).