From the introduction to 'What is String Theory?', presented at the Les Houches Summer School, 1994:
While I was planning these lectures I happened to reread Ken Wilson's account of his early work , and was struck by the parallel between string theory today and quantum field theory thirty years ago. Then, as now, one had a good technical control over the perturbation theory but little else. Wilson saw himself as asking the question ``What is quantum field theory?'' I found it enjoyable and inspiring to read about the various models he studied and approximations he tried (he refers to ``clutching at straws'') before he found the simple and powerful answer, that the theory is to be organized scale-by-scale rather than graph-by-graph. That understanding made it possible to answer both problems of principle, such as how quantum field theory is to be defined beyond perturbation theory, and practical problems, such as how to determine the ground states and phases of quantum field theories.
In string theory today we have these same kinds of problems, and I think there is good reason to expect that an equally powerful organizing principle remains to be found. There are many reasons, as I will touch upon later, to believe that string theory is the correct unification of gravity, quantum mechanics, and particle physics. It is implicit, then, that the theory actually exists, and `exists' does not mean just perturbation theory. The nature of the organizing principle is at this point quite open, and may be very different from what we are used to in quantum field theory.
1. K. Wilson, Rev. Mod. Phys. 55, 583 (1983).