The world is full of experiences that cry out for explanations. Think, for example, of the colors of rainbows and soap bubbles, the vapor trails of high-flying aircraft, the fact that liquid water abruptly changes into solid ice at a certain temperature, the production of lightning and the thunder that follows it in a storm, the beautiful hexagonal symmetry of small snowflakes; all these, and a limitless list of other phenomena, fall within the province of the science of physics. The essence of science in general is the observation and exploration of the world around us with a view to identifying some underlying order or pattern in what we find. And physics is that part of science which deals primarily with the inanimate world, and which furthermore is concerned with trying to identify the most fundamental and unifying principles. The first of these conditions -- restriction to the inanimate world -- separates physics, at least provisionally, from biology; the second separates it from chemistry, which, at least in its theoretical aspects, builds on some specific areas of physics but can ignore some others. Mathematics, of course, although indispensable to the practice of physics, is an entirely different field of study, since it is self-contained and is ultimately independent of observations of the real world.
The subject of this article could be approached in many different ways. One way of obtaining some insight into the nature of physics is to look at the story of how physics has developed from its beginnings until now. That is what this article does, although it makes no attempt to be exhaustive and omits many topics that some might consider important or even essential. Its main purpose is not to offer a chronological survey for its own sake, but just to illustrate how the consistent aim of physics is to relate our knowledge of phenomena to a minimal number of general principles.
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