The subject of the article is the main types of scientific theories and criteria for their validity. There are two fundamentally different types of scientific theories in the structure of any science: phenomenological and transcendental. They differ from each other both in content and in the methods of their construction. Phenomenological theories belong to the empirical level of scientific knowledge, being a generalization of facts and empirical laws of any particular science. Accordingly, the final criterion for the truth of phenomenological theories is the degree to which they correspond to observational and experimental data. Transcendental theories are structural units not of the empirical, but of the theoretical level of scientific knowledge. The immediate objects of this level are no longer observable or material objects, but ideal or purely mental objects. They are not the product of empirical experience, but of thinking and the possibilities available in it for constructing such objects. All mathematical theories, and today also theories of other fields of science, especially fundamental physical theories, are precisely transcendental theories. From the point of view of philosophical ontology, the content of transcendental theories is as objective as the content of phenomenological theories. The only difference between them is that while phenomenological theories describe the properties of the material world, transcendental theories describe the world of objective possibilities. Unlike the material world, we do not observe the world of possibilities, but only think. On the other hand, in ontological terms, the world of possibilities is more fundamental than the world of material objects, since only what is possible in principle always exists in the material world. Everything that is impossible in principle is impossible in material reality. Accordingly, the criterion of truth in transcendental scientific theories is different from that of empirical, phenomenological theories. The three components of the criterion of the truth of transcendental theories are: a) their logical consistency; b) the correspondence of the content of the transcendental theory to the properties and relationships of those ideal objects that are its direct subject; c) the consensus of the disciplinary scientific community regarding the implementation of conditions a) and b) in a particular transcendental theory.