Depression doesn't just happen, it happens for a reason. Sometimes the reason is evident and sometimes it isn't. When we can't establish the reason, the temptation is to consider depression ‘endogenous' (from within) and we assume therefore it must be due to some chemical imbalance in the brain. But what comes first, the chemicals or the cognitions? Or is it more of an interchange where one affects or influences the other, and have we underestimated the capacity of the brain to heal itself?