  We show that Segal spaces, and more generally category objects in an
$\infty$-category $\mathcal{C}$, can be identified with associative algebras in
the double $\infty$-category of spans in $\mathcal{C}$. We use this observation
to prove that "having identities" is a property of a non-unital
$(\infty,n)$-category.
