We walk around the world with a bewildering network of opinions, beliefs and judgments, some of them ephemeral (I think that I won’t need an umbrella today) and some vital (I trust that person).
Why is the sky blue? Why do you believe you are mortal? Why is four twice two? How do you know that George Washington actually existed?
If we have an opinion about such a question, then when we are challenged, we offer what we think counts for evidence. We hope, of course, that our evidence persuades our challenger (i.e., we hope that our rhetoric is up to the task), but, more importantly, we hope that our evidence really justifies our opinion.
It is striking, though, how different branches of knowledge — the humanities, the sciences, mathematics — justify their findings so very differently; they have, one might say, quite incommensurate rules of evidence. Often a shift of emphasis, or framing, of one of these disciplines goes along with, or derives from, a change of these rules, or of the repertoire of sources of evidence, for justifying claims and findings in that field. Law has, of course, its own precise rules explicitly formulated.
1 As is perfectly reasonable, Darwin reserves the word “fact” for those pieces of data or opinion that have been, in some sense, vetted, and are not currently in dispute. The word “evidence” in “On the Origin of Species” can refer to something more preliminary that is yet to be tested and deemed admissible or not. Sometimes, if evidence is firmer than that, Darwin will supply it with an adjective such as “clear” or “plainest”; it may come as a negative, such as “there isn’t a shadow of evidence.”
Even the way the word “evidence” is used can already tell us much about the profile of an intellectual discipline. To take a simple example, consider Charles Darwin’s language in “On the Origin of Species”— specifically, his use of the words “fact” and “evidence” — as offering us clues about the types of argumentation that Darwin counts in...