  We propose a relationship between the cohomology of arithmetic groups, and
the motivic cohomology of certain (Langlands-)attached motives. The motivic
cohomology group in question is that related, by Beilinson's conjecture, to the
adjoint $L$-function at $s=1$. We present evidence for the conjecture using the
theory of periods of automorphic forms, and using analytic torsion.
