Ontological discussions in psychology include the meaning of being human, how psychological competencies contribute to that debate, and how implicit and explicit theories of human nature in the past and present impact psychological theory and practice. Tracing classical theories of human nature, consequences for the science and practice of psychology are debated. Critical theories of human nature are identified and it is argued that at specific points of history particular theories of human nature have dominated, which is not a random phenomenon, but reflects developments in society. Psychologization as well as the fact that many theoretical reflections in academia are based on Western theories, and are hence Western-centric, are discussed. From a critical perspective, the exclusionary dimensions of theories of human nature are presented, drawing on posthuman ideas.