Fear and desire  

You should feel two emotions when approaching a Grotto: fear and desire. You should fear what may be inside, but desire to discover. --Leonardo da Vinci

Full passage:

"I came to the entrance of a great cavern, in front of which I stood some time, astonished and unaware of such a thing. Bending my back into an arch I rested my left hand on my knee and held my right hand over my down-cast and contracted eye brows: often bending first one way and then the other, to see whether I could discover anything inside, and this being forbidden by the deep darkness within, and after having remained there some time, two contrary emotions arose in me, fear and desire--fear of the threatening dark cavern, desire to see whether there were any marvellous thing within it ..." --The Notebooks of Leonardo Da Vinci by Jean Paul Richter