  Exposing a solution to a temperature gradient can lead to the accumulation of
particles on either the cold or warm side. This phenomenon, known as
thermophoresis, has been discovered more than a century ago, and yet its
microscopic origin is still debated. Here, we show that thermophoresis can be
observed in any system such that the transitions between different internal
states are modulated by temperature and such that different internal states
have different transport properties. We establish thermophoresis as a genuine
non-equilibrium effect, whereby a system of currents in real and internal space
that is consistent with the thermodynamic necessity of transporting heat from
warm to cold regions. Our approach also provides an expression for the Soret
coefficient, which decides whether particles accumulate on the cold or on the
warm side, that is associated with the correlation between the energies of the
internal states and their transport properties, that instead remain
system-specific quantities. Finally, we connect our results to previous
approaches based on close-to-equilibrium energetics. Our thermodynamically
consistent approach thus encompasses and generalizes previous findings.
