Proposition VII. An emotion can only be controlled or destroyed by another emotion contrary thereto, and with more power for controlling emotion.
Emotion, in so far as it is referred to the mind, is an idea, whereby the mind affirms of its body a greater or less force of existence than before. When, therefore, the mind is assailed by any emotion, the body is at the same time affected with a modification whereby its power of activity is increased or diminished.
Now this modification of the body receives from its cause the force for persistence in its being; which force can only be checked or destroyed by a bodily cause. In virtue of the body being affected with a modification stronger than itself... the mind will be affected by an emotion contrary to and stronger than the former emotion, which will exclude or destroy the existence of the former emotion.
Emotions are made apparent by changes in the body. Those bodily changes either help us act in the world or hinder us. Persistence in being in the physical world is only controlled by how the body acts and if there is a bodily change which is contrary or stronger than the former one the mind responds with an idea of those changes and drives out the former emotion.