Science is based on two things: measurement and the ability to predict. At the very heart of science is Physics: almost everything we meet in everyday life on earth has SOME properties that modern physics allow us to predict: the predictions can then be verified with astounding accuracy by measurement.
Physics has parts, such as: mechanics which is about Movement and Motion; thermodynamics which is about Temperature and Heat; electricity and magnetism, which are about Charges and Currents and Magnetic attraction; radiation, which includes Light; and very importantly, atomic theory which is about Atoms and Molecules and the properties of gases, liquids and solids.
If we know all of physics we can predict almost all of the behaviour of everyday things here on earth. Even if we only know a small amount, we can understand and predict many really important things: the use of energy in cars and aircraft, house heating and reverse cycle air conditioners, and the effects of diets and exercise!
Some parts of science are not very good at prediction, but to paraphrase Lord Kelvin (1824 to 1907):” if you don’t know how to measure something it is barely science at all”.
What is measurement? Scientists take an approach that I will call “Naïve realism”, namely measurement is an extension of human sensory perception. Sensory perception-sight, touch, smell, taste and hearing, allow us to perceive some of the attributes of physical entities in the external world, and put a name to them. The jargon for this human sensory perception is that the attributes of the entity become the PERCEPT, the mental product of the act of perceiving.
Here is the jargon of measurement corresponding to the jargon of sensory perception. Measurement from the point of view of scientists is perception of the ATTRIBUTES of external objects through the TRANSLATION of the attributes into SIGNALS at an interface, just like the human bodies sensory interfaces: we call these interfaces sensors. The SIGNALS contain information which can be analysed to extract knowledge of the object. If we have an operational definition of how an attribute translates into a signal, and how to extract information about the attribute from the signal, we have a measurement.
This concept of measurement is so important, it is worth saying in different words: MEASUREMENT is possible using known relations of TRANSDUCTION between an {attribute, characteristic, or property} of a {object, system or entity} and a SIGNAL at a {transducer or sensor} interface, that leads to a PERCEPTION of the attribute using an agreed operational definition of how the information is to be extracted. An {attribute, characteristic, property} that is so recognized is called a REFERENT. A signal is a symbolic representation of a value (I hope this finishes the jargon!).
Ideally, a measurement is a 1:1 mapping of a referent to a signal, in a known way, across the transducer interface.
Sounds complicated? Well it is not so bad if we illustrate it with an example: take the perception of the passage of time. One of the earliest measurements of this is the sundial! The transducer here is a circle of stones which you stand in the center of, and read of the time of day with your shadow. For scientists TIME IS WHAT CLOCKS MEASURE, and of course we now have “better” clocks than sundials, or hour glasses, or pendulum clocks, but they are better only in the sense of being more repeatable, more accurate, and covering a wider range of times from the age of the universe to nanoseconds: time is always, and only, what clocks measure.
So why is this “NAÏVE” realism? Well, this correspondence between sensory perception and reality has been controversial for three thousand years of philosophy. Take the flat earth: nobody today thinks the Earth is flat, but actually it IS pretty flat. If you take a piece of the earth as big as a football field, it is flat to about 1 millimeter, so if it was (say) the surface of a lake, or even the ocean, you need something much bigger to perceive its non-flatness with unaided human perception. You need a telescope to see the ships disappearing over the horizon (and the ancient Greeks didn’t have telescopes). It is so flat, the Egyptians did some mighty accurate surveying using water in channels over long distances. So this characteristic of human sensory perception carries over into measurement, and to the extent that later measurements might be more accurate than our current ones, we might find that something we think is completely understood is actually different from our current understanding. This is exactly what happened over the flat earth with Galileo.