There are a large number of disciplines that are interested in the theoretical aspects of the history of thought. Their perspectives and subjects may vary, but fundamentally they have a common research interest: the history of human thinking and its products. Despite this, they are studied in relative isolation. I argue that having different subjects as specific objects of research, such as political or scientific thinking, is not a valid justification for the separation. I propose the formation of a new integrated field of study, the philosophy of the history of thought. Its most fundamental questions can be taken to be: 1) What is the basic theoretical unit in the history of thought? 2) How does change take place and how can it be described? 3) What kind of reasons are there for change? Why is there a change in a particular case? The existing confusions around the commitments and basic vocabulary used in contemporary historiography makes the establishment of this field important. Recognizing that there is such a discipline is necessary in order to enable concentration on the fundamental theoretical issues. It is likely that progress on theoretical questions and better awareness of the implicit commitments would have a positive impact on historical practice.