Defining nature
If nature is a book, it follows that it must contain a language. As Ruskin often reminds us, there are many kinds of language other than speech. Painting and sculpture and architecture are languages as well; to appreciate them properly we have to learn how to read them, just as we do with literature. That this is the case is sometimes obvious. Most people would recognise that Turner's Slave Ship has a 'message' for us. Authorities on medieval art agree that the great French cathedrals are complex 'documents' that convey elaborate scholastic arguments. Yet the Slave Ship, in Ruskin's famous description, is also in some sense a window on the world. The framing of the scene and the integration of its symbols focus and intensify the meaning of the event which it depicts. One might compare Shakespeare's view of the play as something which holds 'the mirror up to nature'.
For meaning to be present in picture or play, there must also in some sense be meaning in nature, and Ruskin asserts that God's judgement on the slavers is 'written upon the sky in lines of blood'.  The writing is partly the calligraphy of paint, partly an inscription by the hand of God as revealed, in effect, to Turner as a prophet.
The distinction between art and nature is not absolute for Ruskin. It is a distinction between two creators of vastly differing power. Nature, quite as much as art, was created to please and instruct. When we create works of art, we imitate and emulate the Creator:
All great art is the expression of man's delight in God's work, not in his own. But observe, he is not himself his own work: he is himself precisely the most wonderful piece of God's workmanship extant. ( Ruskin's italics)
In Modem Painters IV, when Ruskin meditates in a sequence of chapters on the 'materials' of which the earth is made, he even goes so far as to suggest, not just that mountains are works of divine sculpture, but that God has prepared sculptural materials for humans to cut and carve. 'The earth was without form and void', says the book of Genesis. Then the waters were gathered in one place and the dry land appeared. 'The command that the waters should be gathered'', says Ruskin, 'was the command that the earth should be sculptured', Ruskin's italics). A few pages on, when he has embarked on his account of the materials of mountains, he gives these three reasons for the 'appointed frailness of mountains':
The first, and the most important, that successive soils might be supplied to the plains ... and that men might be furnished with a material for their works of architecture and sculpture, at once soft enough to be subdued, and hard enough to be preserved; the second, that some sense of danger might always be connected with the most precipitous forms, and thus increase their sublimity; and the third, that a subject of perpetual interest might be opened to the human mind in observing the changes of form brought about by time on these monuments of creation.
It is an amazingly heterogeneous set of reasons. God here has in mind not only the essential economy of the natural order, but the human need to create and appreciate beauty. A few pages further on still, we find Ruskin reflecting on the kinds of sculpture that are achievable in specific kinds of stone. For example:
The sculptor of granite is forced to confine himself to, and to seek for, certain types of form capable of expression in his material; he is naturally driven to make his figures simple in surface, and colossal in size, that they may bear his blows; and this simplicity and magnitude are exactly the characters necessary to show the granitic or porphyritic colour to the best advantage. And thus we are guided, almost forced, by the laws of nature, to do right in art.
There is here in embryo a Ruskinian doctrine that he never finally formulated in words, though it was to become increasingly more central to his thought. Modernist critics have called it 'truth to material', but Ruskin had already given expression to the concept in, for instance, The Stones of Venice II (1853):
To the Gothic workman the living foliage became an object of intense affection, and he struggled to render all its characters with as much accuracy as was compatible with the laws of design and the nature of his material.
The latter was quite as important for Ruskin as the former. Good sculpture expressed both the subject depicted and the material in which it was executed. Both the leaf and the stone, after all, are products of nature.