The substitution principle (as seen in this book; in italian, sorry)
Let A and B be any two part of an electrical network with voltage and current sources and only resistances, connected by ideal conductors. Suppose the voltage between the conductors is v. Then, in order to study A, B can be replaced with an ideal generator of voltage v.
Any linear electrical network with voltage and current sources and only resistances can be replaced at terminals A-B by an equivalent voltage source Vth in series connection with an equivalent resistance Rth.
There must be something I do not understand.
It seems to me that these statements cannot be both true (or at least, the first one would imply the equivalent resistance in Thevenin's theorem to be zero, and so the theorem loses any meaning.)
Thank you all in advance for the answers!