The author is active as a musician in the field of historically informed performance practice (HIP) which emanated from the 20th century early music movement. Studying of historical sources has a central role, but the problem of how interpretation and selection of source material is affected by art and music ideals of our time, is commonly acknowledged. Georg Joseph Vogler shows in a series of articles in "Betrachtungen der Mannheimer Tonschule" (1778-1781) how Stabat Mater by Pergolesi could, and in his opininon, should be improved. The purpose of this thesis is to investigate how this source can be used from the standpoint of a performing musician: How can I use Vogler?s methods in my playing?The introduction problematizes Vogler?s action in relation to his time, and to ours.