Doubt : a history, by Jennifer Michael Hecht

Here's yet another book I may have to read. From the Washington Post's review:

For instance, Xenophanes, another of the pre-Socratics, argued that the gods of mythology must be human inventions. The Ethiopians posited black gods, while the gods of the red-haired Thracians were, unsurprisingly, red-haired. If horses and oxen had hands and could draw, he dryly remarked, they would draw their gods as great horses and oxen. Xenophanes suggested what Montaigne insisted on 2,000 years later: The exclusive authority claimed by competing religions cannot be taken seriously; their myths derive from obviously local sources, and their truth claims cancel each other out....

Montaigne, as it happens, thought that disagreements among scientists showed that science was as much a cultural construction as religion, and ought therefore to be treated with skepticism.