Changes in bodily power alter mental power
Formal Statement
Whatsoever increases or diminishes, helps or hinders, the power of activity in our body, the idea thereof increases or diminishes, helps or hinders the power of thought in our mind. This is the hinge proposition connecting the conatus to the affects.
In Plain Language
Here is where striving becomes feeling. Because mind and body are parallel (ce-01), any boost to your body's power is simultaneously a boost to your mind's power, and any diminishment of the body's power is a dimming of the mind's capacity. This up-and-down of power is what you experience as emotion. Joy is not a reward dispensed by some inner judge — it is the direct experience of your power increasing. Sadness is not a punishment — it is the felt reality of your power decreasing. Emotion, at bottom, is the mind's awareness of the body's power fluctuations.
Why This Follows
Directly from the parallelism established in ce-01 (II.P7) and the identification of mind with the idea of the body in ce-02 (II.P13-14). Since changes in the body's power of activity are changes in the mind's object, the corresponding idea — the mind itself — undergoes a parallel change in its power of thought.
Emotion is the mind's awareness of increases and decreases in the body's power of acting.
Think of a moment when you felt a sudden surge of energy or vitality. Can you see how that was simultaneously a physical and a mental event, not one causing the other?