  We consider the problem of fair allocation of indivisible items to agents
that have arbitrary entitlements to the items. Every agent $i$ has a valuation
function $v_i$ and an entitlement $b_i$, where entitlements sum up to~1. Which
allocation should one choose in situations in which agents fail to agree on one
acceptable fairness notion? We study this problem in the case in which each
agent focuses on the value she gets, and fairness notions are restricted to be
{\em share based}. A {\em share} $s$ is an function that maps every $(v_i,b_i)$
to a value $s(v_i,b_i)$, representing the minimal value $i$ should get, and $s$
is {\em feasible} if it is always possible to give every agent $i$ value of at
least $s(v_i,b_i)$.
  Our main result is that for additive valuations over goods there is an
allocation that gives every agent at least half her share value, regardless of
which feasible share-based fairness notion the agent wishes to use. Moreover,
the ratio of half is best possible. More generally, we provide tight
characterizations of what can be achieved, both ex-post (as single allocations)
and ex-ante (as expected values of distributions of allocations), both for
goods and for chores. We also show that for chores one can achieve the ex-ante
and ex-post guarantees simultaneously (a ``best of both world" result), whereas
for goods one cannot.
