"Our mind and our delusions are formless and colorless. However, our ignorance believing in true existence is harder than a rocky mountain. Our delusions are harder than steel."
-
Lama Zopa Rinpoche, "The Door to Satisfaction"
And a related one from Rinpoche:
"Happiness and suffering come from your own mind, not from outside. Your own mind is the cause of happiness; your own mind is the cause of suffering. To obtain happiness and pacify suffering, you have to work within your own mind."