The chemist and physicist K. W. G. Kastner is almost unknown today, even though during his lifetime he was among the most respected of German physical scientists. Only the fact that Justus Liebig studied and did his doctorate with him seems to be of (small enough) interest today. Kastner's life and career are shown to be extremely worth analyzing since, like Liebig himself, he succeeded in making his way from being a pharmacist to becoming a university professor. He published numerous textbooks as well as journals, he had a great number of students and he was a popular lecturer. In striking contrast to this are the negative statements of his student Liebig about Romantic Natural Philosophy and about Kastner himself, despite his being of crucial importance for Liebig's career as a teacher and mentor. These critical statements, for which a variety of explanations are known, markedly prejudiced Kastner's reputation for posterity.