The present paper outlines the nature of a three-dimensional ontology and the place of psychological science within this ontology, in a way that is partly similar to and partly different from that of Pérez-Álvarez. The first dimension is the material realities, and involves different levels (physical, chemical, biological, psychological, etc.), where each level builds on a lower level but also involves the development of new emergent properties, in accordance with Bunge's emergent materialism. Each level involves systems, with components, structures and mechanisms, and an environment. This dimension can be studied with natural scientific methods. The second dimension is the subjective-experiential realities, and refers to our subjective perspective on the world. In accordance with Husserl's phenomenology, it is argued that this subjectivity does not exist in the world (i.e., should not be reified as an object among other objects), but represents a perspective on the world that we enter in our capacity as conscious human beings. Essential characteristics of this subjectivity (such as intentionality, temporality, embodiment, and intersubjectivity) can be explored by phenomenological methods. The third dimension is the social-constructional realities, and includes social institutions, norms, categories, theories, and techniques. It is argued that psychological science spans over all three dimensions. Although almost all psychological research by necessity starts from a problem formulation where the subjective-experiential dimension plays an essential role (either explicitly or implicitly), most of present-day psychological research clearly emphasizes the material dimension. It is argued that a mature psychological science needs to integrate all three dimensions.