  In the paper of Keller and Reiten, it was shown that the quotient of a
triangulated category (with some conditions) by a cluster tilting subcategory
becomes an abelian category. After that, Koenig and Zhu showed in detail, how
the abelian structure is given on this quotient category, in a more abstract
setting. On the other hand, as is well known since 1980s, the heart of any
$t$-structure is abelian. We unify these two construction by using the notion
of a cotorsion pair. To any cotorsion pair in a triangulated category, we can
naturally associate an abelian category, which gives back each of the above two
abelian categories, when the cotorsion pair comes from a cluster tilting
subcategory, or is a $t$-structure, respectively.
