The problem of the construction and of the integrability of systems of nonlinear partial differential equations in a multidimensional space is discussed. A proposed algebraic-geometric construction is illustrated for the example of completely integrable equations of the Bourlet type and their generalizations, for which, in particular, Bäcklund transformations aional pseudo-differential operators. They are defined with the help of the ``Feynman measure on phase space'', which is an infinite-dimensional distribution (the kernel of the inverse Fourier transform). Among others the left or the right symbols of pseudo- differential operator are expressed in terms of its kernel and the \(\tau\)-symbol of the product of two pseudo-differential operators is computed using their \(\tau\)-symbols. No proofs are given.