What Is an Emotion? William James’s Revolutionary 1884 Theory of How Our Bodies Affect Our Feelings – The Marginalian

… Long before scientists came to demonstrate how our emotions affect our bodies, James argued that the relationship is bidirectional and that while "bodily disturbances" are conventionally considered byproducts or expressions of the so-called standard emotions — "surprise, curiosity, rapture, fear, anger, lust, greed, and the like" — these corporeal reverberations are actually the raw material of the emotion itself. 

James writes:

Our natural way of thinking about these standard emotions is that the mental perception of some fact excites the mental affection called the emotion, and that this latter state of mind gives rise to the bodily expression. My thesis on the contrary is that the bodily changes follow directly the PERCEPTION of the exciting fact, and that our feeling of the same changes as they occur IS the emotion. Common sense says, we lose our fortune, are sorry and weep; we meet a bear, are frightened and run; we are insulted by a rival, are angry and strike. The hypothesis here to be defended says that this order of sequence is incorrect, that the one mental state is not immediately induced by the other, that the bodily manifestations must first be interposed between, and that the more rational statement is that we feel sorry because we cry, angry because we strike, afraid because we tremble, and not that we cry, strike, or tremble, because we are sorry, angry, or fearful, as the case may be. Without the bodily states following on the perception, the latter would be purely cognitive in form, pale, colourless, destitute of emotional warmth. We might then see the bear, and judge it best to run, receive the insult and deem it right to strike, but we could not actually feel afraid or angry.

https://www.themarginalian.org/2016/01/11/what-is-an-emotion-william-james/