A theory is a set of ideas, hypotheses and explanations that are described in relation to each other. In science, a theory is often a tested model for explaining observations of reality.
Proving, disproving and replacing theories
At least according to the philosopher of science Karl Popper, the correctness of a theory can never be absolutely proven (verified) (it can be made very plausible). This is because no matter how many confirming observations one makes, one can never completely rule out that the next observation will give a different outcome, or that another theory can also explain the observations. If more independent observers make the same observation, agreement can be reached on the accuracy of such observation. Inductivists believed that they could make universally valid statements about reality from a finite number of such observations.
A "good" theory is falsifiable, that is, it makes testable predictions, so that the theory exposes itself to possible refutation in practice. These predictions are tested against new observations. If the predictions match those observations, the theory gains credibility. When refuted, clarity has emerged: the old theory apparently does not work, a new one has to come. The purpose of the explanation and prediction is to understand the phenomenon about which the theory says something. If a theory offers no points of reference in reality (is not testable and therefore cannot be refuted), it is of no use to science: the theory is "not even wrong" (nicht einmal falsch, a statement by the Austrian theoretical physicist Wolfgang Pauli). For a false theory is at least useful if it provokes observations with which the false theory can be disproved. In this way the search for a better one is accelerated.
From the history of science, many theories appear to be replaced by another one at some point. This usually happens when the first theory is falsified: the theory makes predictions that prove incorrect in the experiment. Another reason may be that the new theory is more comprehensive. According to Thomas Kuhn, other non-rational considerations also play a major role. A good new theory is in a sense consistent with its predecessor in that it must also predict the corresponding observations. A disproven theory is often still adhered to by researchers who have worked with it all their lives and only disappears with their death.
A scientific theory can be tested with the help of observations, possibly as a result of experiments. So far, no observations have been made that conflict with the theory of relativity. The same goes for the theory of quantum mechanics.
Some theories have been disproved by experiments. Examples of this are the phlogiston theory and the theory of the workings of the ether.
Examples of theories
Important examples of theories are:
Copernicus's Solar System Theory
James Hutton's Theory of Earth Construction
Thomas Chamberlin's Theory of the Creation of the Earth
Dalton's theory of the structure of matter
Lavoisier's theory of fire
Huygens' theory of light
Albert Einstein's Theory of Relativity
Theory of the cell of Theodor Schwann
Charles Darwin's Theory of Evolution
Louis Pasteur's Theory of Pathogens
Robert Malthus's Population Growth Theory
Freud's theory of the human unconscious
The theory of man by Franz Boas
Karl Marx's Dialectical Materialism
Karl Marx's theory of added value
Bersselaar, V. van den; Philosophy of science in abundance, 2nd edition, Coutinho Bussum 2003.