I cannot but mention among these precepts a new means of study, which, although it may seem trivial and almost ridiculous, is nevertheless extremely useful in arousing the mind to {105} various inventions. It is as follows: when you look at walls mottled with various stains or stones made of diverse substances, if you have to invent some scene, you may discover on them the likeness of various countries, adorned with mountains, rivers, rocks, trees, plains, great valleys and hills in diverse arrangement; again, you may be able to see battles and figures in action and strange effects of physiognomy and costumes, and infinite objects which you could reduce to complete and harmonious forms. And the effect produced by these mottled walls is like that of the sound of bells, in the vibrating of which you may recognize any name or word you choose to imagine. I have seen blots in the clouds and in mottled walls which have stimulated me to the invention of various objects, and although the blots themselves were altogether devoid of perfection in any one of their parts, they lacked not perfection in their movement and circumstance. 50. Obtain knowledge first, and then proceed to practice, which is born of knowledge. [Sidenote: Theory and Practice] 51. Knowledge is the captain, and practice the soldiers. 52. The painter who draws by practice and by the {106} eye, without the guide of reason, is like the mirror, which reflects all the objects which are placed before it and knows not that they exist. 53. Many will consider they can reasonably blame me by alleging that my proofs are contrary to the authority of many men held in great esteem by their inexperienced judgements: overlooking the fact that my works are solely and simply the offspring of experience, which is the veritable master. 54. They who are enamoured of practice without knowledge are like the mariner who puts to sea in a vessel without rudder or compass, and who navigates without a course. Practice should always be based on sound theory; perspective is the guide and the portal of theory, and without it nothing can be well done in the art of painting. [Sidenote: Course of Study] 55. The youth should first learn perspective, and then the measurements of every object; he should then copy from some good master to accustom himself to well-drawn forms, then from nature to acquire confirmation of the theories he has learnt; then he should study for a time the works of various masters, and finally attain the {107} habit of putting into practice and producing his art. 56.