ABSTRACT
There are two widely used approaches to quantum field theory. The first is based on field operators and the canonical quantisation of these field operators. The second approach involves path integrals over classical fields. This chapter develops the idea of path integrals (or functional integrals) in a very intuitive way without any attempt at mathematical precision or rigour. It exploits the analogy between vectors and functions, and between matrices and differential operators on functions. Since very few path integrals can be performed exactly, the chapter concentrates on Gaussian path integrals. These are important in their own right, but much more so because they can be used in approximation schemes when the exact path integral is intractable.
