We present a summary of the topics used in this book: Doubt is the characteristic of every human, everyone makes mistake in his perceptions and more or less understand this mistake. Descartes philosophy begins with doubt and by passing the suspicion process reaches to ensure consistency. We tried to present a brief discussion about possibility and incumbency theorem, since Avicenna. has priority in this topic; his opinion is main center of our discussion. Pivotal item in Avicennas theorem is existence of an external fact independent of human, which is the boundary between philosophy and...