This paper provides the inevitable principle that string theory can only be the theory of everything. The purpose of this paper is to arrive at the beginning of string theory through the assumption of the fundamental principles, and to apply it to the double-slit experiment to confirm the validity of string theory. According to the fundamental principle postulated in this paper, systems can form or break symmetry by free will. Symmetry formation and symmetry breaking coexist based on the indistinguishability of free will and chance. This fundamental principle inevitably leads to the appearance of strings. Through this, phenomena such as the uncertainty principle and the double-slit experiment can be explained simply. It also explains how the formation and collapse of wave functions occur. String theory can explain not only the hyper-microscopic world, but also the microscopic and macroscopic worlds. The fundamental principle is applied as the only philosophical principle of string theory, where all kinds of mathematical techniques are rampant.