The basic point is that any attempt to find an *invariant* meaning
for each of the various verb aspects is a violation of a fundamental
linguistic axiom founded on the work of F. de Saussure. This does
not rule out, however, the possibility that some grammarians may
simply reject this *axiom* of Saussure. Some have rejected it and
many chafe under it.
Arguments about the details of Porter and Fanning tend to obscure
this weakness, which does not manifest itself at the level of
details. This is a weakness in the foundation of their systems not in
refinements.
Clay Bartholomew
Three Tree Point