  We study the structure of two-sided vector spaces over a perfect field $K$.
In particular, we give a complete characterization of isomorphism classes of
simple two-sided vector spaces which are left finite-dimensional. Using this
description, we compute the Quillen $K$-theory of the category of left
finite-dimensional, two-sided vector spaces over $K$. We also consider the
closely related problem of describing homomorphisms $\phi:K\to M_n(K)$.
