  Many physicists, following Einstein, believe that the ultimate aim of
theoretical physics is to find a unified theory of all interactions which would
not depend on any free dimensionless constant, i.e., a dimensionless constant
that is only empirically determinable. We do not know if such a theory exists.
Moreover, if it exists, there seems to be no reason for it to be comprehensible
for the human mind. On the other hand, as pointed out in Wigner's famous paper,
human mathematics is unbelievably successful in natural science. This seeming
paradox may be mitigated by assuming that the mathematical structure of
physical reality has many `layers'. As time goes by, physicists discover new
theories that correspond to the physical reality on the deeper and deeper
level. In this essay, I will take a narrow approach and discuss the
mathematical structure behind a single physical phenomenon - gravitational
interaction between two bodies. The main aim of this essay is to put some
recent developments of this topic in a broader context. For the author it is an
exercise - to investigate history of his scientific topic in depth.
