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From "Augustinian Christian Philosophy", by Alvin Plantinga (on p. 24 of the PDF file):
For in most of the sciences we don't at all have the sort of knowledge we have of the Pythagorean Theorem or the Fundamental Theorem of the Calculus; we don't have anything like the sort of certainty we have in elementary logic and mathematics. Consider physics, for example. First, most of us who know anything about physics know what we know by way of taking someone else's word for it. How do I know that the velocity of light in a vacuum is about 186,000 miles per second? I read it in a physics text, or heard it in a physics class, or saw it in an article in Scientific American. I certainly didn't measure the velocity of light myself, and wouldn't have the faintest idea of how to do so. (I daresay the same is true for you.) How do I know that there are experiments that favor relativity theory over Newtonian mechanics? The same way; I learned it in a physics class. I didn't myself perform those experiments involving muon decay or the rapid transport of cesium clocks, or the measurement of parallax. Indeed, the same goes for most physicists: most of them, so far as I know, haven't performed those experiments either; most of them learned about them in class or from a physics journal. As a matter of fact, even those who did perform the experiments had to take a great deal on the authority of others: that the velocity of the plane transporting the cesium clock was in fact thus and so, that the plane flew the relevant distance and the right course, and so on. Anyone who makes an advance in science obviously stands on the shoulders of others, taking an enormous amount on their say-so--for example, how the earlier experiments relevant to his project turned out.
I post this passage in part because most of us, especially those of us who regard science as the highest or truest form of knowledge or way of pursuing knowledge, vastly underestimate the importance of testimony (including documentary evidence) in our knowledge and belief systems. Anything we believe on the say-so of someone else, whether it is an informal recommendation on a new car purchase, or an article in cosmology with a new measurement of the cosmic background radiation left over from the Big Bang, or Xenophon's account of the life of Socrates, is something we believe not because of our empirical observations of the matters in question, but because we trust the testimony we receive on the matter. Implicit in our believing a thing on the basis of testimony is the acknowledgement of the authority of the testifier at least as far as the thing believed is concerned.