All the cases of perspective are understood by means of the five terms of the mathematicians, that is: point, line, angle, surface, and body. Of these, the point is unique in its generation, and this point has no height, nor breadth or length or depth; whence it is concluded to be indivisible and to have no place. The line is of 3 natures, that is: straight, curved, and sinuous. It is that which has no breadth nor height or depth; whence it is indivisible, except along its length; its ends are 2 points. The angle is the termination of 2 lines at a point, and they are of 3 natures, that is: right, acute, and obtuse. Surface is that which is called the boundary of bodies, and has no depth; indeed in that depth it is indivisible, like the line or the point, and it is divided only by length or breadth. These are of as many kinds as there are bodies that create them. Body is that which has height, breadth, length and depth, and in every direction is divisible. These bodies are of infinite and various forms. Visible bodies are only of 2 natures, of which the first is without figure, or [without] any distinct or defined extremities; and even if there are any, they are imperceptible, so that the colour is scarcely known. The second species of visible bodies is that whose surface bounds and defines their figure. The first species, which is without surface, is that of those bodies which are thin or liquid, that easily pour out and mingle with other thin bodies, as mud with water, as fog or smoke with air, as the element of air with fire and other similar things, which with their extreme parts have mixed with the bodies near them, so that through such mixture their boundaries are confused and imperceptible, whence they are found to be without surface because they enter one body into another; and so we say such bodies to be without surface. The second species of body is divided into two other natures, that is: transparent and opaque. The transparent is that which on every side shows its whole and occupies nothing behind itself, as glass, crystal, water and the like. The second species of bodies whose surface displays and bounds its figure is called opaque. This is the one of which it behoves to treat at length, since from it derive infinite cases.