The beginning of science lies in observation. When we look at the world around us, we see that many things happen. Apples fall off trees, the leaves of some trees change color in the autumn, the tides come in and go out, chickens lay eggs, and so on. One job of scientists is to describe and classify what hap- pens and what exists. But scientists also wonder why some things happen rather than others. Maple trees change color in the fall, and spruce trees do not, but why? Chickens lay eggs, and monkeys do not, but why? A sphere of wood floats in water, and a gold sphere does not, but why? And why does gold float when pressed into the shape of a boat? These questions ask for explanations.

To provide an explanation, scientists often give arguments of the kind discussed in Chapter 1. The event to be explained is derived from a general principle plus a statement of initial conditions or particular facts. For exam- ple, given the general principle that a solid sphere floats in water if and only if it is less dense than water, and also given the particular facts that wood is less dense than water, whereas gold is denser than water, we can explain why a wooden sphere floats in water and a gold sphere does not.

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Scientists often seek deeper explanations by asking why certain general principles themselves are true. The principle that a sphere floats in water only when it is less dense than water can be explained as an instance of the more general principle that anything floats only when it displaces more than its own weight in water. This broader principle not only explains why a wooden sphere floats in water but also why a piece of gold will float when molded into the form of a boat. This broader principle is in turn explained by deriving it from even more basic principles about gravity and the mutual repulsion of molecules. A larger scientific theory is thus used to explain not only why par- ticular things happen but also why certain general principles hold.


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