The article analyzes the status of metaphysics in relation to other sciences, especially the sense and reasons behind its priority in the system of sciences, as conveyed in the works of Thomas Aquinas. The question of what comes first in the system of sciences has led to an exploration and justification of the criteria behind this priority. According to Thomas Aquinas, metaphysics is justly considered to be the first philosophy: on the one hand it is occupied with what comes first (...) in the ontological order – the first causes of being, on the other hand, other sciences rely on it for their first principles. The article critically analyzes both substantiations of the idea of being first. The substantive criterion is questioned by the introduction of revealed theology into the system of sciences accepted by Aquinas; revealed theology is also occupied with what comes first, and does so with greater authority than metaphysics. The article focuses on the analysis of main doubts concerning metaphysics’ methodological criterion of priority: the idea that metaphysics, in relation to other sciences, is in a sense first and functions as a determinant, while also being last and determined by these very sciences. Metaphysics is first, as other sciences draw from it their first principles, and last, as it utilizes facts established by other sciences which come first in the process of knowledge acquisition. Hence the charge that Aquinas’ argumentation concerning metaphysics’ priority is circular in nature. The article analyzes various aspects of this difficulty and offers suggestions on how to overcome them. (shrink)