Kukai wrote:

Kukai was the founder of the monastery at Mount Koya and introduced Esoteric Buddhism to Japan. He lived from 774 to 835 CE.

I never tire of admiring the old pine trees and moss-covered stones of Mount Koya. Clear streams on the mountain never cease flowing with compassion. Beware taking pride in the superficial poisons of secular fame and profit.

When your mind is steeped in darkness, all that you encounter becomes evil.  If your mind’s eye is bright, everything you encounter becomes a treasure.

 He proposed a meditation on the letter A, which has a moon shaped character:

It implies “no”, “not”, and “never”.

The first sound humans make when thy open their mouths is this. Without this sound, there can be no language. It is the mother of all speech and the basis of countless characters.

Furthermore, it signifies that all things are transient, neither born nor unborn.

It is not only the Buddhist teachings, but all teachings that build upon this letter.