This is the well ordering principle as it is given in Thomas Koshy's number theory book -

Every non empty set of positive integers has a least element.

For example the set {17,23,5,18,13} has a least element 5.

By virtue of the well ordering principle, the set if positive integers in well-ordered. You may notice that the set of negative integers is not well ordered.

I didn't understand the statement - "You may notice that the set of negative integers is not well ordered".

A set of negative numbers also has a least number. Or is it that the well ordered principle is confined to only positive numbers and that's the reason the above statement holds good?