 Our affects are divided into actions and passions. When the cause of an event lies wholly in our own nature—more particularly, our knowledge or adequate ideas— and we are the “adequate cause” of it, then it is a case of the mind acting. On the other hand, when something happens in us the cause of which lies outside of our nature, then we are passive and being acted upon. Usually what takes place, both when we are acting and when we are being acted upon, is some change in our mental and physical capacities, what Spinoza calls “an increase or decrease in our power of acting” or in our “power to persevere in being”. All beings are naturally endowed with such a power or striving. This conatus, a kind of existential inertia, constitutes the “essence” of any being. “Each thing, as far as it can by its own power, strives to persevere in its being.” An affect just is any change in this power, for better or for worse. Affects that are actions are changes in this power that have their source (or adequate cause) in our nature alone; affects that are passions are those changes in this power that originate at least partly outside of us.