  A special integro-differential formula (SIDF) of the class of Green's
theorems is established for a pair of functional forms (FFs) defined in a real
affine Euclidean space of any given dimension n>=1 (nDRAES); one of the FFs is
the Green FF for a Poisson equation in the unbounded nDRAES, and the other one
is an arbitrary twice-continuously-differentiable time biased (retarded or
advanced) FF. The SIDF is an identity, which depends on n, but which is not
associated with any specific differential equation for the arbitrary FF. If,
however, the latter is assumed to satisfy an inhomogeneous wave equation (IWE)
in nDRAES then the SIDF expresses the forced retarded or advanced solution of
the IWE if and only if n=3. Based on this fact and also on the pertinent
properly defined causality principle, it is concluded that an nDRAES can be
regarded as a receptacle of Nature, i.e. as a receptacle of matter along with
the principle of special relativity and also along with all metamorphoses,
which occur to matter in time in the irreversible direction from past through
present to future - metamorphoses that are called physical, chemical,
biological, etc. processes - if and only if n=3.
