Methodological problems (I shall use "methodology of science" alternatively with "philosophy of science") ranked as most important in the opinion of the Lvov—Warsaw School. This was due to the immense prestige of science in the School. There were also great expectations that the methodology of sciences would make philosophy more scientific. That was why problems in the philosophy of sciences were very willingly taken up — from the very elementary (the concept and types of reasoning) to the much advanced ones (probabilistic logic and its applications in quantum mechanics). The works of the School in the philosophy of science were quantitatively immense, probably greater than those in other branches of philosophy, and its systematization and presentation would require a separate monograph. My intention is to report on those methodological studies which help one to acquire orientation to the views of the Lvov—Warsaw School on the essential issues in the philosophy of science: reasoning, induction, probability, and justification.