To understand the enormity of Galileo’s experiments in mechanics we need to appreciate something of the Aristotelian science of movement against which he so decisively revolted. The essential containing framework of Aristotle’s view is that of an ordered cosmos in which everything has its natural place. A still and unmoving earth is at the centre of nine spheres which revolve round it in perfect circles. The earth, as its name implies, is made of earth and is a place of imperfection, while the heavenly spheres are made of a quite different crystalline substance to which imperfection is unknown. Because the earth is still, the default state of all the things it contains, which are made of different mixtures of the four elements of earth, air, fire and water, is to be at rest. But obviously they are often not, so Aristotle has to have a theory of motion. He divides movement into two kinds, natural and violent. Natural motions on earth are caused by the circular movements of the planets. It is planetary influence that makes trees grow from saplings and birds fly. But there is also violent motion imposed on natural things by human beings, who often move them from their natural place. They raise stones with windlasses to build towers, propel ships through water with oars, prevent arrows from dropping immediately to the ground by firing them through the air with bows. But as soon as this interference stops, objects, as can be easily seen, seek to return to their natural place as soon as they can because they have a natural appetite to do so. If the windlass breaks the stones fall, if the oarsmen stop rowing the ship stops moving, when the force lent by the bow peters out the arrow falls to the ground.
Objects move naturally back to their natural places in the different ways that they do because of the different mixtures of the four elements that compose them. The elements come in two pairs, heavy and light. Earth is the more heavy and water the less, fire is the less light and air the more. This is why really earthy things like stones try to drop back to the centre of the earth in the most direct and determined way, while water just trickles down to the lowest level it can. Flames rise upwards, while the more airy smoke that the flames produce rises higher still. Every earthly motion, in Aristotle’s view, has to have an efficient cause, that is, it has to be immediately and physically moved by something else. He thinks this because the way he does physics is to go out there and take a look. Have you ever seen a thought firing an arrow, or a wish lifting up a stone? Thus he doesn’t like vacuums, abhorred by nature he thinks, in a famous phrase, just as much as by Aristotle himself. For one thing Aristotle believes that things seek to return to their natural station as quickly as possible, though usually impeded. But in a vacuum there would be no impediments and arrival would therefore be instantaneous, which he thinks impossible. Furthermore, there would be no backwards and forwards or up and down, so things wouldn’t know where to go. Finally, nobody has ever seen anything moving in a vacuum, it just doesn’t happen. For the same kind of reason, because he has such a physical attitude to physics – it’s all about taking a look, observing what actually goes on out there – he thinks that physics is nothing to do with mathematics and geometry. Maths and geometry don’t go on in the world, they happen in your head.
Given his strong emphasis on efficient cause in motion, actual physical things pushing other things until they move, Aristotle has to deal with the problem of projectiles. If you throw a ball up in the air you lose immediate contact with it but it still keeps on moving.
Aristotle’s answer is that you yourself may have lost contact with the ball but the air hasn’t. What happens is that when you throw the ball you give a mighty great push to the particles of air immediately in contact with your hand, and they in turn push further particles of air which push other particles which… push the ball. As the force you gave to the air becomes weaker the ball slows until it reaches a point where it overcomes the violent motion that you had imparted and seeks to return to its natural place on the ground. It is the motion of the air that causes projectile movement.