start using <em>subtraction</em> (the inverse operation of addition),
you start running into negative numbers,
which are numbers outside the set of natural numbers.<span class="intersentencespace"></span> If the only mathematical operations you will ever use are <em>addition</em> and <em>subtraction</em>,
then the set of integers <span class="inline_math">\( \mathbb{Z} = \{ \ldots, -2, -1, 0, 1, 2, \ldots \} \)</span> will be sufficient.<span class="intersentencespace"></span> Think about it.<span class="in