No mind really can only ever be a temporary state, not because it causes brain damage, but because for as long as you have to function in the outside world, you will have to think. Mushin is a natural outcome of reaching some understanding of anatman (no-self). Recognising that there is no essential self should lead to not being attached to any particular thought that arises.

Although there are periods where the mind is totally free from any thought whatsover--it is only temporary. One of the points of something like shamatha mediation is to recognize how thoughts constantly bubble to the surface. This may slow down at some point, but there will always be some thoughts. Even the Buddha had thoughts (if not he wouldn't have said anything!). So the trick here is to not be attached to your thoughts, either by labeling them as 'just a thought' or some other method, and then letting it dissolve again. Then for moment the mind is empty, then another thought will appear. For some, this moment becomes longer and longer, but there will always be a thought to bubble forth.

WATCH OUT FOR THAT BUS!

--Chris
_________________________
"Seek not to follow in the footsteps of the men of old; seek what they sought."
--Basho