  The canonical formalism of QFT as introduced in the previous section is only applicable in the case of free fields since the inclusion of interaction leads to infinities (see the historical part). Since little of realistic models can be solved exactly, perturbation theory makes up a large part of most publications on QFT. The importance of perturbative methods is understandable realizing that they establish the immediate contact between theory and experiment. Although the techniques of perturbation theory have become ever more sophisticated it is somewhat disturbing that perturbative methods are difficult to avoid. Some have argued this is a matter of principle. (And on the other hand, physicists have been astonishingly creative in developing “toy models— that can be solved exactly, precisely to escape perturbation theory.)  One reason for unease is that perturbation theory is felt to be rather a matter of (highly sophisticated) craftsmanship than of understanding nature. Accordingly, the corpus of perturbative methods plays a small role in philosophical investigations of QFT. Two recent exceptions are Fraser (2018) and Passon (2019). What does matter, however, is in which sense the consideration of realistic interactions affects the general framework of QFT. An overview about perturbation theory is given in section 4.1 (“Perturbation Theory—Philosophy and Examples”) of Peskin &amp; Schroeder (1995).