The t function, introduced by the 'Kyoto school' as a central element in the description of soliton equation hierarchies, is identified with the determinant of a family of linear operators solving linear, constant-coefficient PDE in the hierarchy variables, for the Kadomtsev-Petviashvili (KP) hierarchy. For Gel'fand-Levitan-Marchenko integral operators, the t function is the Fredholm determinant; we give a new proof of this fact. We show further that the determinant of a suitably chosen family of finite-dimensional matrices is a t function which gives rise to rational solutions of the K P equation. Finally. we prove the 'vertex operator identity' for T functions in the Fredholm determinant case.