Do you run because you are afraid, or are you afraid because you run?
What it argues
James inverts the common sense: the bodily change comes first and the feeling is the perception of that change. We do not tremble because we are afraid; we are afraid because we tremble. The claim is that affect is information about the body, not a verdict about the world.
The probe
At the next strong reaction, find the body event before the label: chest, jaw, gut, hands. Write the event first, the emotion word second.
The source
What Is an Emotion?, Mind 9 — William James, 1884. The link goes to the text or to a catalogue record for it, not to a summary. This one is the text itself.