When I was studying psychology, one of the points we discussed was whether or not biological responses followed psychological reactions or whether the psychological reactions were the result of biology. That is, when you are scared into a "fight or flight" reaction, do you first experience the release of adrenaline and then feel fear, or do you feel fear then the adrenaline is released?
At the time, I recall thinking that it didn't really matter all that much. I also thought that it was an odd question to ask as I was sure that feeling preceded neurochemical release. As I look back on the question, I realize that there may have been a deeper issue to explore. If biology precedes psychological responses, then our emotions and responses are mere slaves to our internal chemical processes. The fact that people often experience mood swings in accord with things like hormones and blood sugar levels definitely supports this idea.
On the other hand, I don't know that we can say we are slaves to our biology. As I sit here typing, I'm slowly being overtaken by post-meal sluggishness. I know my tired feeling is being set off by biology and I wonder to what extent I can reject that response and push my body and mind not to act on this all too common response to having recently eaten. Soldiers have been trained to effectively maintain their core body temperature in freezing water. They can suppress their need to shiver and keep their trunk warm so that they will not experience a slowing of their heart rate so rapidly. If we can "think" our way out of biological response, surely this would encourage the idea that our feelings and mental application are masters of biology rather than the other way around.
I wonder if, in the end, the entire question of which comes first is at its heart a debate over a mindset that reduces man to the sum of his chemistry or one that elevates him to master over his particular biological domain. One viewpoint renders us powerless and the other grants us a great deal of power over ourselves. What is much more interesting to me than which is true is what would motivate a particular person to embrace one theory or another. These days, I'm much more inclined to view us as capable of influencing our biology should we apply ourselves to doing so, particularly when it comes to mastering emotions and responses to stimuli.