## Thinking Mathematically (6th Edition)

$1-4=-3$ 1 and 4 are natural numbers, but -3 is not.
Natural numbers are the counting numbers: 1, 2, 3, ... The set of natural numbers is not closed with respect to subtraction, as can be seen in the example below: 1 and 4 are natural numbers. However, $1-4 = -3$, which is not a natural number.