## Thinking Mathematically (6th Edition)

For an empty set $A$ not to be a subset of a set $B$, there has to be at least one element of $A$ that isn't also an element of $B$, and as $A$ has no elements, there isn't one. Therefore, we can say that the empty set is a subset of every set.
For an empty set $A$ not to be a subset of a set $B$, there has to be at least one element of $A$ that isn't also an element of $B$, and as $A$ has no elements, there isn't one. Therefore, we can say that the empty set is a subset of every set. The answer is derived from the definition of an empty set and the definition of a subset.