The human spirit. This abstract embodiment of what it means to exist as a human encompasses limitless strength.
The worst thing that can happen to someone is to have their spirit broken, yet it can not be broken without our consent. This means that no matter what hard ships that may come our way, if we never give in to despair and repression then it can never truly hold power over us. This goes with anything that is of an abstract nature be it fear, anger or self depreciation.
We must keep in mind however that being limited stretches the spirit thin and leaves ourselves open to the risk of a damaged spirit. If we only believe certain things to be true and hide from the possibilities it makes the possibilities that much more destructive. "Plan for the worst and hope for the best." This does not make you masochistic or a fatalist, just realistic and open minded. After all, if you have hope you can not live in despair at the same time.
Samurai were taught that every day they should consider their death. Be it burning, laceration, seppuku or any other possibility of demise. This is what enabled them to face death so easily, they faced it every day.
Consider the possibilities and strengthen your spirit. Even though life can be harsh, you will find that everything that can go wrong usually doesn't despite preparedness and that will already make the world that much brighter in contrast.
In truth, life is life and things happen, but the only reason something might be viewed in a negative fashion is due to outlook and considering it unthinkable or depressing. There is good in all things as there is bad, look closely, I'm sure you will see it. That is the balance of the world.
So do not neglect the spirit when strengthening the mind and body!