Since their transition from theology to modern science in the 17th century, basic terms such as identity, person, subject, ego have been associated with considerable difficulties. This work documents the complications using the identity theory approaches of the pre-Kantian era as an example, which although failing theoretically in their monologic-monadic orientation were however able to affirm that the concept of identity was indispensable in practical terms. In addition it proves that there was already a concept of empirical psychology in the works of Leibniz and that interpretations of the key term apperception will not be able to avoid this in the future. The study concludes with an introduction to Tetens, who can justifiably be seen as the one who did the groundwork for and also was a contributor to the Copernican Revolution.