When we fly in an airplane above the clouds, we realize that the sun is always shining even when it is cloudy and rainy below. In the same way, when we cease to hold on to our identity, our ego, we begin to see that the nonexistence of ego is a powerful, real, and indestructible state of being. We realize that, like the sun, it is a continuous situation which does not wax or wane. That state of being is called vajra nature.
From “Sacred Outlook,” in The Heart of the Buddha, page 137.