  This paper proves the existence of viscosity solutions of path dependent
semilinear PDEs via Perron's method, i.e. via showing that the supremum of
viscosity subsolutions is a viscosity solution. We use the notion of viscosity
solutions introduced by Ekren, Keller, Touzi and Zhang, in whose work all
smooth processes which are tangent in mean are considered as test functions. We
also provide a comparison result for semicontinuous viscosity solutions, by
using a regularization technique. As an interesting byproduct, we give a new
short proof for the optimal stopping problem with semicontinuous obstacles.
