  An examination is made of the way in which particles emerge from linear,
bosonic, massive quantum field theories. Two different constructions of the
one-particle subspace of such theories are given, both illustrating the
importance of the interplay between the quantum-mechanical linear structure and
the classical one. Some comments are made on the Newton-Wigner representation
of one-particle states, and on the relationship between the approach of this
paper and those of Segal, and of Haag and Ruelle.
