Imagine that you are sitting naked on the ground, with your bare bottom touching the earth. Since you are not wearing a scarf or hat, you are also exposed to heaven above. You are sandwiched between heaven and earth: a naked man or woman, sitting between heaven and earth. Earth is always earth. The earth will let anyone sit on it, and earth never gives way. It never lets you go—you don't drop off this earth and go flying through outer space. Likewise, sky is always sky; heaven is always heaven above you. Whether it is snowing or raining or the sun is shining, whether it is daytime or nighttime, the sky is always there. In that sense, we know that heaven and earth are trustworthy.
From "The Genuine Heart of Sadness," in Shambhala: The Sacred Path of the Warrior, Shambhala Library Edition, page 28.