<<The doctrine of the natural place of things is based on that of the four elements: fire, air, water and earth. Earth tends downward and fire upward, with air tending toward a level between fire and water, and water toward a level between air and earth. Where things move naturally depends on the proportion of their elements, so that a piece of wood floats because it contains air, for example. The earth is a sphere, so the natural place of the element earth is a central sphere, surrounded by a shell of water, which in turn is surrounded by a shell of air, which finally is ringed by a shell of fire. >>
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Oh I think I get it. Thanks bobpursley. So the piece of wood has nothing to do with staying in level with the earth?
Aristotle followed a theory of composition, believing the world was made of four elements--earth, water, air, and fire (though he later adds a fifth element, quintessance, in order to try and explain planetary movement.) Earth is the heaviest material with the greatest association with our earth; water is lighter than earth but heavier than air; air is lighter than water but heavier than fire; and fire is the lightest of the four. These four elements and their properties were used to explain rudimentary phenomena in Aristotle's time that he and other scientists did not understand such as gravity. Since gravity--or why things fell towards the earth --was a concept not fully understood, Aristotle explained it as such: objects made primarily of earth-materials, such as stone, have a greater association with the earth, and as such are drawn to the earth because they wish to be reunited with their natural place of order (remember Aristotle is a teleological thinker and sees things as having a set place, purpose, and exactly one form of movement.) This is why stones fall to the earth when picked up and dropped. Because earth-materials are heavier than water-materials, stones will fall through water when dropped in a pond or lake on their way to be reunited with their natural place of order. On the other hand, items made of other materials, such as fire- or air-materials, will float because they are naturally lighter and because of their natural place being skyward.
Now take wood. Wood is made of earth-materials, but floats on water. Aristotle observed wood and noticed that wood is porous--covered with little holes that allows air to move through. He deduced that wood is primarily earth-material, but has a secondary property of air-material (and a tertiary property of fire-material since it is "dry," but that matters a little less here.) The air-material part of it is lighter than the earth-material, and affects it when the wood is dropped in water. Since the earth-material aspect is its primary composition, it falls towards the earth when dropped. However since its air-material composition is lighter than water, when wood is dropped in water wood will float.
Boats were built out of wood in Aristotle's time, and his four elements system of the composition of the world was widely accepted as fact(until after Copernicus and Galileo came into the picture.) The reason why boats float, according to Aristotle, is the same reason as to why wood floats--because its air-properties make it lighter than the water on which it sits.