Georg Cantor was the first among scientists to begin to study infinity, which was of philosophical interest, with mathematical precision. His innovative approach to mathematics was embodied in set theory, and he formulated counterintuitive concepts of different kinds of infinity. Until the works that were published by scientists at the end of the 19th century and became fundamental contributions to science, infinity, following the scientific tradition dating back to Aristotle, was understood as a useful convention. Cantor's courage cost him dearly: his ideas were harshly rejected by many of his contemporaries, which probably caused his mental illness and premature death. Note. OCR: Due to the way Hebrew is displayed in expressions, aleph(X) has been replaced by X.