"For a variety of reasons function spaces are very useful in many investigations and applications. To a large extent, modern mathematics is concerned with the study of general structures. The essential thing is finding the right generalization. Insufficient generality can be too restrictive and a great deal of generality may result in a situation where little can be proved and applied. The space introduced by Banach, especially pointing out completeness, attests to his genius; he hit the traditional nail on the head."

Krzysztof Ciesielski (2007). On Stefan Banach and some of his results. Banach J. Math. Anal., 1(1), 1-10.