The Way is shown in five books concerning different aspects. These are Ground, Water, Fire, Wind, and Void.
The body of the Way of strategy from the viewpoint of my Ichi school is explained in the Ground book. It is difficult to realise the true Way just through sword-fencing. Know the smallest things and the biggest things, the shallowest things and the deepest things. As if it were a straight road mapped out on the ground, the first book is called the Ground book.

Go Rin No Sho (1645)