What is the Warrior’s Creed and where did it originate?

The following is dubbed the Warrior’s Creed, found on just about every martial arts page on the Internet.

The poem is heavy on religious principle, very Zen in nature, and attributed to an anonymous samurai from the 14th century. It might even be legitimate.

I have no parents: I make the heavens and Earth my parents.
I have no home: I make awareness my home.
I have no life or death: I make the tides of breathing my life and death.
I have no divine power: I make honesty my divine power.
I have no means: I make understanding my means.
I have no magic secrets: I make character my magic secret.
I have no body: I make endurance my body.
I have no eyes: I make the flash of lightning my eyes. I have no ears: I make sensibility my ears.
I have no limbs: I make promptness my limbs.
I have no strategy: I make “unshadowed by thought” my strategy.
I have no design: I make “seizing opportunity by the forelock” my design.
I have no miracles: I make right action my miracles. I have no principles: I make adaptability to all circumstances my principles.
I have no tactics: I make emptiness and fullness my tactics.
I have no talents: I make ready wit my talent.
I have no friends: I make my mind my friend.
I have no enemy: I make carelessness my enemy.
I have no armor: I make benevolence and righteousness my armor.
I have no castle: I make immovable mind my castle.
I have no sword: I make absence of mind my sword.