Observation and experimentation are the two characteristic features of science. Prior to Galileo, scientists based their theories on speculation and aesthetic preferences. He broke away from this tradition. Ever since, it has become a standard method of science to accept theoretical predictions only if repeated observations and precise measurements provide enough evidence. Indeed as Lord Kelvin so aptly put it. "When you can measure what you are speaking about and express it in numbers, you know something about it, when you cannot express it in numbers, your knowledge is of a meager and unsatisfactory kind."