It is evident that it is not by their taste or by their odour that the natures of natural bodies are known, for if they were recognised by their colours, all white bodies would have the same nature, and everything of the same colour or taste would be of the same nature; for we see that snow and dough and fresh cheese have the same colour although each of them has a different nature. We see again that, although parsley and aloes agree in pungency, they are not of the same nature since one is sharp and the other mild; and in the same way we speak of smell. Consequently, not by their taste or smell or touch, is the nature of the natural bodies known, but by their positions and motions.
The ill-informed declare that the firmament changes62 in parts, or in its own entirety, with length of time, although that change is not apparent to us, as we see gold, iron, the body, jacinth and many other precious stones becoming discoloured with age and length of time, and changing in size, colour, taste and smell with length of time, although we are not aware of that change when it is in progress, on account of the great extent of time during which it is carried on.
I say to them in answer, that everything that is beneath the moon in the sphere of the four elements undergoes change, and that change is of two kindscomplete and partialand those two changes concern growth and decay, and the complete change is more evident than the partial. Every body that is changed, undergoes that change in quantity or quality, and the actions of the body make that change evident to us; for when an unsound body becomes sound, and a sound unsound, it is the actions of that body that make that
p.160change evident to us. So, when a heavy body becomes light, the actions of the same body make that change evident to us. Thus, when a slow body becomes swift or a swift slow, it is the swiftness or slowness of that body that makes that motion evident to us.
Thus if the firmament were increased or decreased, its actions would manifest that change to us. For were it augmented and extended, and the constellations placed further from us than they are, we should see then a smaller quantity of them than we do now observe, and there are many of them which we see now, that we should not see then. Consequently, when it would have closed in around the earth, the quantity of the stars would seem to us more prodigious then than now, and then we would see many stars that we did not see before. Now, since no one ever has seen these changes in the firmament, for if he had, it would be found written; it is sure and certain, that the firmament suffers neither increase nor decrease, and has neither contracted nor expanded63.
Similarly if the firmament inclined to its right or to its left, or forwards or backwards, or if it moved up or down from the position in which it is, the centre, i.e., the earth, must necessarily change with it but the earth cannot leave its own position, since it has no place to go; for every place around it is full of other bodies; and since two bodies cannot on any account occupy the same place, the earth cannot leave its own position to join other bodies, and consequently the firmament must be in a permanent, immovable, immutable position around the earth for ever. If it were said that the change of the firmament resulted from the failure or decrease of its substance, as the human body becomes weakened by ill-health, in the same way the motion of the firmament would be weakened, as is the walk of a sick man. In the same way the hours and seasons would be changeda thing that has never happened, for they have always had the same course and have today, and will have it for ever.
Thus, since the substance, or quality, or quantity, or position, or motion, or time of the firmament, or of the rising, or setting, or concealment, or revealing of the stars are not changed, and since that has never happened, and never will happen, it must necessarily be one firm immovable system that it had, has, and will have, as long as He who created it desires, and it must have a nature other than the nature of the four elements.