  We are answering the question why 4-dimensional space has the metric 1+3 by
making a general argument from a certain type of equations of motion linear in
momentum for any spin (except spin zero) in any even dimension d. All known
free equations for non-zero spin for massless fields belong to this type of
equations. Requiring Hermiticity(This is a generalization of an earlier work
which shows that without assuming the Lorentz invariance -which in the present
work is assumed- the Weyl equation follows using Hermiticity.) of the equations
of motion operator as well as irreducibility with respect to the Lorentz group
representation, we prove that only metrics with the signature corresponding to
q time + (d - q) space dimensions with q being odd exist. Correspondingly, in
four dimensional space, Nature could only make the realization of 1+3
dimensional space.
