The Passivity of Passion

Spinoza maintains that insofar as we experience passions of a sort I just described, what we're doing is experiencing the way that external things interact with us. So in what sense they passive then? The passivity lies in the idea that you're being acted on by something and you're responding to it. Spinoza describes the kind of idea that I have of you or indeed anything else that acts on me as an inadoput idea.