The development of philosophical ideas throughout history has sometimes been assisted by the use of handcrafted instruments. Some paradigmatic cases, such as the invention of the telescope or the microscope, show that many philosophical approaches have been the result of the intervention of such instruments. The aim of this article is to show the determining role that stringed musical instruments with frets had in the crisis and generation of philosophical paradigms. In fact, just as the observations of the moon with the telescope broke more than a thousand years of Aristotelian hegemony, the fretted string instruments, predecessors of the guitar, played a central role in the collapse of one of the most influential approaches in the history of Philosophy: Pythagorism. We focus on the fundamental hallmarks of Pythagorism and on how, during the 16th century and from the fretted string instruments, the mathematical-musical notion of equal temperament emerged, which from the middle of the 19th century will be established as the prevailing philosophical-musical paradigm of the West.