  In this article, we give an abstract characterization of the ``identity'' of
an operator space $V$ by looking at a quantity $n_{cb}(V,u)$ which is defined
in analogue to a well-known quantity in Banach space theory. More precisely, we
show that there exists a complete isometry from $V$ to some $\mathcal{L}(H)$
sending $u$ to ${\rm id}_H$ if and only if $n_{cb}(V,u) =1$. We will use it to
give an abstract characterization of operator systems. Moreover, we will show
that if $V$ is a unital operator space and $W$ is a proper complete $M$-ideal,
then $V/W$ is also a unital operator space. As a consequece, the quotient of an
operator system by a proper complete $M$-ideal is again an operator system. In
the appendix, we will also give an abstract characterisation of ``non-unital
operator systems'' using an idea arose from the definition of $n_{cb}(V,u)$.
