Observation
One important aspect of sceince is obseravtion of events. But observation required imagination, for scientists can never include everything in a description of what they ovserve. Hence, scientists must make judgements about what is relevant in their observations. As an example, let us consider how two great minds, Aristole and Galileo, interpreted motion along a horizontal surface. Aristotle notied that objects given an initial push along the ground (or on a tabletop) always slow down to a stop. Consequently, Aristotle believed that the natural state of an object is to be at rest. Galileo, in his reexamination of horizontal motion in the early 1600s, imagined that if friction could be eliminated, an object given an initial push along a horizontal surface would continue to move indefinitely without stopping. He concluded that for an object to be in motion was just as natural as for it to be at rest. By inventing a new approach, Galileo founded our modern view of motion, and he did so with a leap of the imagination. Galileo made this leap conceptually, without actually eliminating friction.
Theories are creations
Observation and careful experimentation and measurement are one side of the scientific process. The other side is the invention or creation of theories to explain and order the observations. Theories are never derived directly from observations. They are inspirations that come from the minds of human beings. For example, the idea that matter is made up of atoms (the atomic theory) was certainlly not arrived at because someone observed atoms. Rather, the idea sprang from creative minds. The theory of relativity, the electromagnetic theory of light, and Newton's law of universal gravitation were likewise the result of human imagination.
The great theories of sceince may be compared, as creative achievements, with great works of art or literature. But how does science differ from these other creative activities? One important difference is that science requires testing of its ideas or theories to see if their predictions are borne out through experiment.
Testing a theory
Although the testing of theories can be considered to distinguish science from other creative fields, it should not be assumed that a theory is "proved" by testing. First of all, no measuring instrument is perfect, so exact confirmation cannot be possible. Furthermore, it is not possible to test a theory for every possible set of circumstances. Hence a theory can never be absolutely "proved." In fact, theories themselves are generally not perfect - a theory rearely agress with experiment exactly, within experimental error, in every single case in which it is tested. Indeed, the history of science tells us that long-held theories are sometimes replaced by new ones. The process of one theory replacing another is an important subject in the philosophy of science; we can discuss it here only briefly.
Theory Acceptance
A new theory is accepted by scientists in some cases because its predictions are quantitatively in much better agreement with experiment than those of older theory. But in many cases, a new thoery is accepted only if it explains a greater range of phenomena than does the older one. Copernicus's Sun-centered theory of theuniverse, for example, was originally no more accurate than Ptolemy's Earth-centered theory for predicting the motion of heavenly bodies. But Copernicus's theory had consequences that Ptolemy's did not, such as predicting the moonlike phases of Venus. A simpler (or no more complex) and richer theory, one which unified and explains a greater variety of phenomena, is more useful and beautiful to a scientist. And this aspect, as well as quantitative agreement, plays a major role in the acceptance of a theory.
An important aspect of any theory is how well it can be quantitatively predict phenomena, and from this point of view a new theory may often seem to be only a minor advance of the old one. For example, Einstein's theory of relativity gives predictions that differ very little from the older theories of Galileo and Newton in nearly all everyday situations. Its predictions are better mainly in the extreme case of very high speeds close to the speed of light. In this respect, the theory of relativity might be considered as mere "fine-tuning" of the older theory. But quantitative prediction is not the only important outcome of a theory. Our view of the wold is affected as well. As a result of Einstein's theory of relativity, for example, our concepts of space and time have been completely altered, and we have come to see mass and energy as a single entity (via the famous E=mc2). Indeed, our view of the world underwent a major change when relativity theory came to be accepted.