Is a thought simply the sum total of the activity of all the neurons in the brain? No, just as a message is not simply the placement of a sequence of letters which follow the rules of grammar. How do those letters stand in relation to that message? How does the activity of neurons stand in relation to a thought? Was Descartes right, or are we our brain?
What causes your depression: serotonin or what you experience? The answer: the question is invalid! Both play a role, and are interconnected. You cannot say that one of these is the cause.