I often read that we must judge arguments or claims based on their own merits, rather than on the quality of the person presenting them. This is fine in realms such as logic or everyday life, where we can all have access to the relevant information, but how does this play out in complex domains, such as science? For instance, suppose I am reading two books on the health effects of different nutrients, such as animal fat. One author claims all animal fat is harmful, the other claims that some animal fats, such as fish fat, is fine in moderation. Both cite studies supporting their views, but one author is a spokesperson for PETA and the other is a senior researcher at a well-known university. As somebody who doesn't have access to biology laboratories to conduct experiments, and who perhaps doesn't have the time to read every source cited, critique every study made and read every attack made on both author's views, what is the best thing for me to do? Should I simply decide not to believe anything at all? It seems this is the only rational course of action, as to take a stand based only on two equally plausible-seeming books and two author biographies would be irrational. Yet simply saying "Oh well, I can't know for sure!" seems rather depressing.

Your question touches on two much-discussed philosophical topics: the epistemology of (expert and non-expert) testimony and the epistemology of disagreement. You can find accessible discussions of those topics here and here. I won't opine about those topics in general except to say that the academic credentials and scholarly independence of anyone making a scientific claim are highly relevant and worth checking.

Fortunately, the particular example you gave is fairly tractable: the issue "Is all animal fat harmful even if consumed in moderation?" In fact, the debate you referred to is even more tightly focused: "Is fish fat, even in moderation, harmful?" Thanks to that tight focus, you can narrow your search to sources answering that particular question. The web makes such a search easier than ever before. Start with the most recent peer-reviewed articles you find, because they're supposed to take account of and respond to earlier articles; their conclusions will be summarized in their abstracts. In other words, find out if you can what the very latest research says about the health effects of fish fat in moderation. You may find that the evidence makes you form an opinion one way or the other (subject, of course, to correction by the results of future research). Or you may find that inconclusive evidence makes you suspend judgment on the issue, which is often the right attitude anyway.

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