Amplifiers homework problem

1. The problem statement, all variables and given/known data

2. Relevant equations

$$dI = \frac{\partial I}{\partial V_{GS}} dV_{GS} + \frac{\partial I}{\partial V_{DS}}dV_{DS}$$

3. The attempt at a solution

I was just wondering why $$\small V_{G}$$ is at 0. I have solved the entire question (there are more parts :P), but I could only do it once I was told that $$\small V_{GS} = - V_S + V_G = - V_S$$. I am having a hard time getting my head around that.
Shouldn't $$\small V_{G}$$ simply be $$\small V_{i}$$?

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