 James Fraser (2016) identifies three problems for perturbative QFT. (1) The rigor problem: perturbative QFT is not mathematically rigorous which makes it difficult to analyze and interpret. (2) The consistency problem: perturbative calculations rest on the interaction picture existing, but Haag’s theorem seems to show that the interaction picture does not exist. (3) The justification problem: renormalization lacks physical motivation and appears ad hoc. James Fraser argues that (1) and (2) do not pose severe problems for perturbative QFT because it is not attempting to build continuum QFT models. It is building approximate physical quantities – not mathematical structures that are to be interpreted as physical systems.