Probability: The Old and New Aspects of Janus-Facedness

ABSTRACT Probability is likened to Roman god Janus whose one face is looking to the past and another is looking to the future. The reason of this analogy is that probability, on one hand, is a measure related to objective reality and on the other hand it is a measure related to subjective reality. The phrase of Janus-faced probability expresses this analogy. The referred phrase can be accepted for the period of 1654-1837 by the philosophers and mathematicians living in that period; because in that period the concept of probability exhibit the property being a double measure related two realities. This state, however, changes after probability is divided into two during the period of 1837-1843. After the end of the mentioned period, the concept of probability is considered as a measure related to only one reality by both people who think ？Real probability is subjective probability？ and those who think ？Real probability is objective probability？. Not a measure related to double realities. Attitude towards approving one type of probability and denying the other type continues until the year 1945. A different idea rises from Rudolf Carnap who is the central thinker of the Vienna Circle. According to him, there are two probabilities and both of them are useful from the point of science. It is because there is no need for one to win over another. What is necessary is cooperation between these two probabilities. One of the first applications of cooperation is done by Rudolf Carnap’s student, Carl Gustav Hempel, while delineating scientific explanation and prediction activities. Wesley C. Salmon and others follow him. After all these, probability once again has become Janus-faced. It should be noted that, however, the old and new aspects of Janus-facedness are not the same anymore. This study aims to exhibit the mentioned situation