Physiology tells us that when the brain is not functioning well, for whatever
reason, for instance because of insufficient oxygen in its blood, our
mind suffers, and so what we call our spirit;
this is not my opinion, but a scientific fact.
At the same time having a depressed, or deeply sad mood for an extended period
of time could lead to serious problems to the immuno-system, or even to acts that would harm or even put an end to our body.
The causes and effects are circular and reverberate, like in a hall full of mirrors.
Obviously the 'spirit' could be described in term of neuronal processes in the brain, but this description still seems like a daunting task, so we keep this artificial distinction.