  The far-infrared (FIR) regime is one of the few wavelength ranges where no
astronomical data with sub-arcsecond spatial resolution exist. Neither of the
medium-term satellite projects like SPICA, Millimetron nor O.S.T. will resolve
this malady. For many research areas, however, information at high spatial and
spectral resolution in the FIR, taken from atomic fine-structure lines, from
highly excited carbon monoxide (CO), light hydrids, and especially from water
lines would open the door for transformative science. A main theme will be to
trace the role of water in proto-planetary disks, to observationally advance
our understanding of the planet formation process and, intimately related to
that, the pathways to habitable planets and the emergence of life. Furthermore,
key observations will zoom into the physics and chemistry of the star-formation
process in our own Galaxy, as well as in external galaxies. The FIR provides
unique tools to investigate in particular the energetics of heating, cooling
and shocks. The velocity-resolved data in these tracers will reveal the
detailed dynamics engrained in these processes in a spatially resolved fashion,
and will deliver the perfect synergy with ground-based molecular line data for
the colder dense gas.
