The PhD project is embedded in the research project Optimal Proofs funded by the Netherlands Organisation for Scientific Research led by Rosalie Iemhoff at Utrecht University. The project in mathematical and philosophical logic is concerned with formalisation in general and proof systems in particular. Its mathematical aim is to develop methods to describe the possible proof systems of a given logic and establish, given various criteria of optimality, what the optimal proof systems of the logic are. Its philosophical aim is to develop general criteria for faithful formalisation in logic and thereby distinguish good formalisations from bad ones. The mathematical part of the project focuses on, but is not necessarily restricted to, the (non)classical logics that occur in Computer Science, Mathematics, and Philosophy, while the philosophical part of the project also takes into account domains where formalisation in logic is more rare. The research of the PhD lies mainly within the mathematical part of the project, but can, depending on the interests of the candidate, be extended to the philosophical part of the project.