A heightened sense of the observation of nature is one of the chief delights that have come to me through trying to paint. I accumulated in those years so fine a surplus in the Book of Observance that I have been drawing confidently upon it ever since.

All bodies together, and each by itself, give off to the surrounding air an infinite number of images which are all-pervading and each complete, each conveying the nature, colour and form of the body which produces it.

Thus it is with a deaf and dumb person who, when he sees two men in conversation - although he is deprived of hearing - can nevertheless understand, from the attitudes and gestures of the speakers, the nature of their discussion.

The painter strives and competes with nature.

The painter who is familiar with the nature of the sinews, muscles, and tendons, will know very well, in giving movement to a limb, how many and which sinews cause it; and which muscle, by swelling, causes the contraction of that sinew; and which sinews, expanded into the thinnest cartilage, surround and support the said muscle.

Shadow partakes of the nature of universal matter. All such matters are more powerful in their beginning and grow weaker towards the end, I say at the beginning, whatever their form or condition may be and whether visible or invisible. And it is not from small beginnings that they grow to a great size in time; as it might be a great oak which has a feeble beginning from a small acorn. Yet I may say that the oak is most powerful at its beginning, that is where it springs from the earth, which is where it is largest.