The Philosophical Motivation for Proof-Theoretic Harmony Guido van der Knaap Abstract: This thesis presents, discusses, and evaluates the philosophical motivations for proof-theoretic harmony - one of the central concepts of logical inferentialism - and relates them to the corresponding formal notions. It will be argued that the principle of innocence manages the objections against the philosophical motivations for harmony in the most satisfying way. Since the principle of innocence is formulated regarding the deductive system as a whole, this strongly suggests that the formal harmony requirement needs to be a global one. The considerations regarding the corresponding formal notions endorse this view, in particular it will be shown how local constraints fail to rule out constants such as quantum disjunction and bullet, the proof-theoretic variant of the Liar sentence. The further aim of this thesis is to emphasize the role of the structural rules and the context of a rule, both made explicit by the sequent calculus, for the inferentialistic behaviour of a logical constant.