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In the past, various criteria or measures for explanatory power have been proposed. In particular, one hypothesis, theory, or explanation can be said to have more explanatory power than another about the same subject matter
- if more facts or observations are accounted for;
- if it changes more "surprising facts" into "a matter of course" (following Peirce);
- if more details of causal relations are provided, leading to a high accuracy and precision of the description;
- if it offers greater predictive power (if it offers more details about what should be expected to be seen and not seen);
- if it depends less on authorities and more on observations;
- if it makes fewer assumptions;
- if it is more falsifiable (more testable by observation or experiment, according to Popper).
Recently, David Deutsch proposed that theorists should seek explanations that are hard to vary. By that expression, he intended to state that a hard-to-vary explanation provides specific details that fit together so tightly that it is impossible to change any detail without affecting the whole theory.
The philosopher and physicist David Deutsch offers a criterion for a good explanation that he considered to be possibly just as important to scientific progress as learning to reject appeals to authority and falsifiability. To Deutsch, the aspects of a good explanation and more are contained in any theory that is specific and "hard to vary". He believes that criterion to help to eliminate "bad explanations" that keep adding justifications, and can otherwise avoid ever being truly falsified. An explanation that is hard to vary but does not survive a critical test can be considered to be falsified.
Deutsch takes examples from Greek mythology. He describes how very specific, and even somewhat falsifiable theories were provided to explain how the god Demeter's sadness caused the seasons. Alternatively, Deutsch points out, one could have just as easily explained the seasons as resulting from the god's happiness, which would make it a poor explanation because it is so easy to arbitrarily change details. Without Deutsch's criterion, the 'Greek gods explanation' could have just kept adding justifications. The same criterion, of being "hard to vary", may be what makes the modern explanation for the seasons a good one. None of the details about the earth rotating around the sun at a certain angle in a certain orbit can be easily modified without changing the theory's coherence.
Relation to other criteria
It can be argued that the criterion hard to vary is closely related to Occam's razor: both imply logical consistency and a minimum of assumptions.
The philosopher Karl Popper acknowledged it is logically possible to avoid falsification of a hypothesis by changing details to avoid any criticism, adopting the term an immunizing stratagem from Hans Albert. Popper argued that scientific hypotheses should be subjected to methodological testing to select for the strongest hypothesis.
- David Deutsch, "A new way of explaining explanation"
- David Deutsch (2011), The Beginning Of Infinity", ch1, The Reach of Explanations
- Ray S. Percival (2012), The Myth of the Closed Mind: Explaining why and how People are Rational, p.206, Chicago.
- Karl R. Popper (1934), The Logic of Scientific Discovery, p.20, Routledge Classics (ed. 2004)