The attached file is differential pair sensing an input common-mode change.
Here is an paragraph in my book:
What happen if Vin,cm = 0? Since the gate potential of M1 and M2 is not more positive than their source potentials, both devices are off.

Now I want to ask about the rule to know that "the gate potential of M1 and M2 is not more positive than their source potentials". The gate potentials is 0 V but how can I know that the source potentials of M1 and M2 is larger than 0 V? Why these source potentials can be negative?

 

How could anything in the circuit be negative? Presumably Vdd > 0V and, by definition, ground IS 0V.

 

How could anything in the circuit be negative? Presumably Vdd > 0V and, by definition, ground IS 0V.

I know that but is there any rules for you to say for sure that
I know it is easy for you but I am struggling to understand the most basic things.

 

The statement:

"the gate potential of M1 and M2 is not more positive than their source potentials" [at cut-off]

does not preclude the possibility of them [gate and source] being at equal potential.

So it is quite possible that both source and gate could be at the same value which would include 0V [as in your particular example] or even a non-zero voltage value within the range of 0 to +Vcc.

In any event your premise regarding the source potentials being less than zero is invalid for this circuit for the logical reason Wbahn states. On the same matter, it's unclear whether you are actually asking if it is possible for the overall circuit to function correctly [as a differential amplifier] with a common mode input of 0V; which on reflection would lead one to conclude it clearly cannot.