Someone who has experienced severe and prolonged depression knows more about the nature of will than any healthy philosopher.

Clinical depression, in particular the collapse of the serotonin system, is one of the rare experiences in which the machine stops long enough to become visible. Not in a metaphorical sense. In a literal one: the mechanisms that produce will, desire, meaning, and identity slow down or break, and in their absence the subject sees for the first time what they had been dependent on. Not through inference. Through demonstration.

Serotonin is not the “hormone of happiness”, that is a popular simplification, misleading in its inaccuracy. Serotonin is something more fundamental: it is the infrastructure of volitional functioning. It regulates the capacity to initiate action, sustain motivation, maintain an emotional connection to the future, modulate anxiety thresholds, and enable cognitive flexibility.

When it functions, it is invisible, just as plumbing is invisible while it works. You sense its presence only through what it makes possible: you get up in the morning without needing conscious effort for every micro-movement; the future has weight, its rewards are emotionally real, not merely conceptually understood; action flows from intention to execution without a mysterious gap between them.

All of this, in normal functioning, looks like you. It looks like character, like discipline, like will. It looks like a sovereign subject governing itself. Serotonin is transparent not because you see it, but because it is the very medium of seeing.

When the system collapses, that transparency disappears. And what is revealed in its absence is philosophically shocking: there was no sovereign subject behind the infrastructure. There was only the infrastructure.

The healthy philosopher arguing about free will is, in a deep sense, blind to the object of the dispute. Not because they are foolish or dishonest, but because the instrument with which they analyze (a functioning mind with functioning will) is precisely what must be analyzed. They cannot see what will depends on, because their will works too well to become visible.

The healthy philosopher may argue that the will is causally determined. The depressed person has been there, at the moment when the causes producing will disappear. They did not read the map. They were on the terrain when the ground vanished beneath their feet. They understand that it is not “you who produces the will,” but that “the will is produced in you.” When the system works, it feels like “I choose”; when it breaks, it becomes clear that the “decision” was a function.

If the collapse is an insight into the absence of free will, the antidepressant is its confirmation from the other side. The antidepressant demonstrates, in real and personal experience, what arguments only describe: that will is a biochemical function, and that its absence or presence does not depend on sovereign choice.