See this circuit, from Sedra & Smith microelectronics, 6th edition, page 287:
It says that the resistance between the gate and source looking into the source is 1/gm, but that the resistance between the gate and source looking into the gate is infinite. Why? What does "looking into" even mean and what difference does it make?
According to my understanding, whether you look into the source or into the gate, the resistance between G and S is 1/gm. If you put apply a voltage between G and S and measure the current with Ohm's law, you would find that R is 1/gm.
There must be something I don't understand.
EDIT: Here's another related thing I don't understand. See this circuit:
It says that Rin is vi/-i. I can kind of see where does this expression come from but I don't know the formal definition of Rin. Why is there a – in front of the i?