This article goes to the foundations of Statistical Inference through a review of Carnap's logic theory of induction. From this point of view, it brings another solution to the attention of the scientific community in the light of certain recent results, and, in particular, how it relates to a new formulation of the Principle of Indifference. As is known, the idea of a probabilistic logic of inductive inference based on some form of the Principle of Indifference always retained a powerful appeal. Keynes recommended a modified version of the principle in order to achieve this aim. Carnap followed Keynes in this attempt of creating a purely logical theory of induction. However, up to now all modifications of the principle failed. In this paper a modified version of the Principle of Indifference is provided without generating paradoxes and inconsistencies. Besides, a general criterion of assignment of prior probabilities in case of initial ignorance is suggested, thus providing a re...