What follows was prompted by Nicholas Wade's New York Times Book Review (October 11, 2009). My comment is about scientific DATA, FACTS, LAWS and THEORIES.
Cognitive Structure of Science
a) Any recorded piece of information, about a natural phenomenon, is DATA. It can be the result of an observation or an experiment; it can be qualitative or quantitative.
b) A piece of data recognized as valid, by most practitioners in a given field, becomes a FACT. Facts are data that are reproducible; any scientist using the same protocol is expected to obtain the same data; at a specified level of reproducibility. Science is not mathematics; that is why the level of reproducibility, for example 90%, should be reported.
c) A LAW is a generalization of facts. I am thinking about Kepler's Laws (how planets move), about Mendeleyev's Law (how elements are ordered in a chart), Faraday's Law (how changing currents can be induced), etc.
d) A THEORY is an explanation of a law. I am thinking about Newton's gravitational theory, about Maxwell's equations, about quantum mechanics, etc. Distinctions between laws and theories (and between facts and theories) are worth recognizing. Unfortunately, this is not always done.
e) Theories are based on assumptions which may or may not be valid. Assumptions can be based on facts, on laws, or on intuition.
f) Scientific theories are validated not only on the basis of their logical (mathematical) correctness but also on the basis of their ability to guide to discoveries of unknown facts. Scientific theories evolve; some theories are more powerful, more general, and more elegant, than others.
g) The concept of 'absolute truth,' often used by mathematicians and theologians, does not belong to science. Wade wrote: "A theory, however strongly you believe it, inherently holds a small question mark. The minute you erase the question mark, you've got yourself a dogma."
h) A hypothesis is a tentative idea to be confirmed. It is a common instrument of human thinking. Hypothesizing is used by all those who collect data, who validate data, who generalize from available data, and by those who explain facts of nature. Tentative assumptions are made by all thinking people, not only by physical scientists.
I am a retired nuclear physicist and my current research is a very controversial field, known as "cold fusion." A very large number of papers, published in that field, is downloadable from the library at http://www.lenr-canr.html Nearly all experimental reports, in my opinion, are data rather than fact. I am leading an attempt to turn one piece of data into a fact. Five people are independently using the same experimental protocol. Will our data be more or less the same as those reported last year? This remains to be seen.