The cause which moves the water through its springs against the natural course of its gravity is like that which moves the humours in all the shapes of animated bodies.125 [With a drawing of the heart showing veins and arteries.] O writer, with what words will you describe with a like perfection the whole arrangement of that of which the drawing is here? For lack of knowledge you will describe it confusedly so as to convey but little perception of the true shapes of things; and deceiving yourself you believe that you can satisfy the listener completely when you speak of the ﬁgure of anything that has body and is surrounded by surfaces. I recommend that you do not cumber yourself with words unless you are speaking to the blind, or if you wish to demonstrate to the ears with words rather than to the eyes of men speak of things of substance or of nature and do not busy yourself in making enter by the ears things which have to do with the eyes for in this you will be far surpassed by the work of the painter. With what words can you describe this heart without ﬁlling a whole book? Yet the more detail you write concerning it the more you will confuse the mind of the hearer. And you will always need commentators or to go back to experience, and this with you is very brief and only deals with a few things as compared with the extent of the subject concerning which you desire complete knowledge.126 Of the human eye The pupil of the eye changes to as many diﬀerent sizes as there are diﬀerences in the degrees of brightness and obscurity of the objects which present themselves before it. . . . In this case nature has provided the visual faculty, when irritated by excessive light, with the contraction of the pupil and here nature works like one who, having too much light in his habitation, blocks up the window more or less according to necessity, and who, when night comes, throws open the whole of this window in order to see better. . . .