  The observed general time-asymmetric behavior of macroscopic systems --
embodied in the second law of thermodynamics -- arises naturally from
time-symmetric microscopic laws due to the great disparity between macro and
micro-scales. More specific features of macroscopic evolution depend on the
nature of the microscopic dynamics. In particular, short range interactions
with good mixing properties lead, for simple systems, to the quantitative
description of such evolutions by means of autonomous hydrodynamic equations,
e.g., the diffusion equation.
  These deterministic time-asymmetric equations accurately describe the
observed behavior of {\it individual} macro systems. Derivations using
ensembles (or probability distributions) must therefore, to be relevant, hold
for almost all members of the ensemble, i.e., occur with probability close to
one. Equating observed irreversible macroscopic behavior with the time
evolution of ensembles describing systems having only a few degrees of freedom,
where no such typicality holds, is misguided and misleading.
