S. L. Rubinstein, whose centennial is celebrated this year, created an unique philosophical-psychological concept of a person, his activity and psychics, freedom and creativity, cognition and speech, ideality, subjectivity and objectivity. He suggested and systematically worked out activity notion in psychology and then generalized it on the basis of philosophical determinism principle: external causes act through internal conditions. The history of S. L. Rubinstein's development of this most important principle is described in the article. It is shown that A. N. Leontiev's formula "internal through external" is not an alternative to Rubinstein's determinism principle but only one of its aspects.