Recourse to "sample space" or "attribute space" is a requisite for probability calculation in some belief-related philosophical interpretations of probability such as classic, logical, and epistemological interpretations. On the basis of the principle of indifference, the probability of original elements is assumed equal in such calculations. There are, however, two questions about this principle to be discussed in philosophical debates about probability. First, what is the epistemological validity of this principle, and second, how to resolve paradoxes resulting from applying this principle? None of the questions have met sufficient answers till now. The author, in this article, reviews various interpretations of probability in order to show which one of them is in need of this principle. Then he justifies the principle of indifference based on the principle of causality and with reference to knowledge by presence both in epistemological probability and in general. The main part of the article deals with eight paradoxes along with different suggestions for their resolution. The author finds all of them flawed, and suggests a new answer on the basis of an epistemological interpretation of probability.