This volume may be of interest for all those who wish that philosophy had a scientific character. As an adherent of the Polish Lvov-Warsaw Philosophical School, the author of this collection of papers endeavours to clarify some basic notions of epistemology, ontology and psychology of cognitive acts, such as judgment, existence, being etc. In his investigations he refrains from unnecessary rejection of common-sense knowledge but at the same time searches for suitable patterns in contemporary sciences. Regarding formal logic as a fundamental tool for the precise expression and justification of thoughts, the author tries to clear logic from ontological commitments, shows how to construct logic of norms and how to use safely different definitions in research works. The book presents a new conception of antinomies and an innovatory approach to realistic epistemology. Moreover, some applications of logical methods are illustrated by examples of semantical analyses of the general notion of similarity and the biological concept of homology.