Thus it happened that the goal of erecting a pure electromagnetic
field theory of matter remained unattained for the time being,
although in principle no objection could be raised against the
possibility of reaching such a goal. The thing which deterred
one in any further attempt in this direction was the lack of any
systematic method leading to the solution. What appears certain
to me, however, is that, in the foundations of any consistent
field theory, there shall not be, in addition to the concept
of field, any concept concerning particles. The whole theory must
be based solely on partial differential equations and their
singularity-free solutions.