  The two-dimensional Ising model is representable as a lattice free-fermion
field theory in terms of the integral over anticommuting Grassmann variables.
The exact solution in a zero magnetic field then follows by evaluating Gaussian
fermionic integral for partition function. It is argued that by switching on of
a non-zero magnetic field the non-local and non-Gaussian fermionic terms appear
in the action, which rather imply approximating methods of the analysis.
