: Kant speaks of our capacity to be “self-determining […] in certain […] laws holding firm a priori”. Here the “laws” refer to the categories of freedom introduced in KpV. The categories of freedom, then, are necessary for self-determination. I first explain how Kant employs the notion of determination in his theoretical philosophy. I then explain how the notion is utilized also in his practical philosophy, particularly in connection to the act of determining the forms of willing that make morality possible. I conclude by answering the question, Why are the categories of freedom necessary for self-determination? Understanding why the categories of freedom are required for self-determination will also help clarify the nature and function of Kant’s categories of freedom, which have puzzled many scholars.