The author's purpose is to offer an exposition of the principle of indispensable reason introduced by a Wolff scholar Ch. F. Baumeister (1709-1785) and expounded in his 'Institutiones metaphysicae' (1736). At the time when Baumeister was working on his book the principle of indispensable reason was a frequent object of commentaries and it led to a vehement discussion (haelischer Streit). Baumeister was blamed for proposing universal determinism and fatalism, destroying thereby free agency and morality. Regardless of the validity of these criticism, Baumeister had given a paragon exposition of the principle of indispensable reason by relating it to the principle of consistency, by examining its drivability and essential properties. He was specifically interested in showing its relativity (non-absoluteness), provability, generality and intelligbility.