Study of Gneiss Rock, Glenfinlass by John Ruskin. Pen, brown ink, ink wash (lamp-back) and bodycolour, 47.7 x 32.7 cm. Ashmolean Museum, Oxford, England [Source: Wikimedia]
" And whether you are drawing a piece of Greek armour, or a hawk's beak, or a lion's paw, you will find that the mere necessity of using the hand compels attention to circumstances which would otherwise have escaped notice, and fastens them in the memory without farther effort."
John Ruskin (in “Lectures on art”)