Network theorem origin
The Norton theorem is a direct consequence of the superposition principle which arises from the linearity of the Maxwell's equations. 

Suppose we have the following general network

&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;

where i=1,2,...,N , and j=1,2,...,M. 

Hence we can express the current I as 

$$I=\sum_{i=1}^N \alpha_i V_i+\sum_{j=1}^M \beta_i A_i + G_N V=A_N+G_N V,$$

which is the Norton theorem.