Posted by alexandra_k on February 22, 2005, at 3:39:39
It is said that Plato set the following problem for his students: What uniform and ordered motions must be assumed for each of the platnets to account for its apparently irregular motions across the night sky? This has been known as Plato's problem. It was a significant event in the intellectual history of human civilization, not so much because this challenging problem had engaged some of the best minds of Europe for almost two millennia, but because it was probably the first-ever significant suggestion that observed phenomena should be regarded as mere appearances, to be explained in terms of postulated reality. In response, Johannes Kepler (1571-1630), following the lead of Ptolemy, Copernicus, et al., proposed that the planets' real motions were in the form of ellipses around the sun. It was the combination of these elliptic motions of the planets and the earth's own motion that gave rise to the observed retrograde loops. What is the logic of this type of explanation?
Currently, the most popular analysis of scientific explanations is probably Carl Hempel's deductive-nomological model of explanations, which can be illustrated with the following simple example.
To explain why the block of wood started to move, one can say that it is because the block was pushed by a force. According to Hempel, this explanation takes the form of a valid deductive argument:
The block was pushed by a force
All forces produce motion
Therefore, the block started to move.Here the explanandum (the conclusion) is said to be explained by an initial condition (the first premise) through a law of nature (the second premise). This is Hempel's model of explanation in a nutshell.
Is Kepler's explanation a deductive-nomological expalantion (D-N explanation)? I would say not. In D-N explanations, the explanandum is affirmed to be true. Indeed, the purpose of the deduction is to demonstrate that the explanandum has to be true, assuming the truth of the premises. In contrast, in Kepler's explanation, those retrograde motions were denied to have ever existed. If deduction is involved in Kepler's explanation, it would not be the deduction of a statement to the effect that the planets move in retrograde loops. What is being deduced would be more like:
It is *observed* that the planets move in retrograde loops.
Kepler's explanation is an example of what can be called REALITY VERSUS APPEARANCE EXPLANATIONS. The logic of this type of explanation deserves close study. The true driving force of the advancement of science is not Hempel's D-N explanations, as commonly thought. It is reality versus appearance explanations that have brought science to what it is today.
Hung, H.-C. (forthcoming) Beyond Kuhn: Scientific Explanation, Theory Structure, Incommensurability and Physical Necessity
poster:alexandra_k
thread:461662
URL: http://www.dr-bob.org/babble/write/20050211/msgs/461662.html