Mathematical description of the Real World phenomena is objective if it is independent on the observer. That is, it is possible to reconcile observation of phenomena into a single coherent description of it. This requirement was pointed out by Galileo Galilee (1564-1642), Isaac Newton (1643-1727), Albert Einstein (1879-1955) in the context of mathematical description of the mechanical movement: “The mechanical event is independent on the observer “.

The majority of mathematical descriptions reported in the literature are objective. However, there are descriptions, which are nonobjective. In this paper, it is shown that mathematical description of elastic phenomena appearing in a vibrating guitar string, which uses Caputo or Riemann-Liouville fractional order derivatives having integral representation on a finite interval, is nonobjective.