A typical scientist has no responsibility other than to explain how a natural event occurred. However, when a philosopher asks about the conditions under which a scientist’s explanation is true, he is, in fact, raising an ultimate question, the concept which Karl Popper used for the first time. Answering this question requires that no elements are neglected in the explanation, and no significant factors in the explanation are overlooked. In other words, in explainingphenomenon, at any level of its explanation, there should be no remainder. These requirements can be achieved through the full explanation. In the present article, by drawing on concepts such as theory-ladenness of observation, underdetermination of theory by evidence, and the role of models and metaphors in developing a scientific theory, it is illustrated thatcomplete explanation includes both a scientific explanation and personal explanation. A personal explanation comprises mental properties such as belief, desire, and intention, which are irreducible to physical properties. Therefore, we cannot provide a personal explanation while restricting ourselves to scientific methods. Consequently, it is argued in this article first, the personal explanation is irreducible to a scientific explanation. Second, the personal explanation is inevitable in order to provide a full explanation. Third, (methodological) naturalists claim that the ultimate judgment of what is natural and unnatural is possible only by scientific inquiry. Finally, accepting these three premises entails the inability of a naturalist to answer ultimate questions.