The concept of linear systems given by Mesarovic and Takahara (1975) is generalized to the model of general systems introduced by Ma and Lin (1987). Examples are given to show the difference between the old and the new concepts of linear systems. The fundamental theorem of linear systems theory proved by Mesarovic and Takahara (1975) is verified in a new way. An equivalent description of the concept that an input-output system is a linear system, and the stability and uniform stability of input-output linear systems are studied.