I connect two increasingly popular ideas in social epistemology—group knowledge and epistemic extension—both departures from mainstream epistemological tradition. In doing so, I generate a framework for conceptualizing and organizing contemporary epistemology along several core axes. This, in turn, allows me to delineate a largely unexplored frontier in group epistemology. The bulk of extant work in group epistemology can be dubbed intra-group epistemology: the study of epistemically salient happenings within groups. I delineate and attempt to motivate what I dub inter-group epistemology: (...) the study of epistemically salient happenings between groups and other subjects and entities. (shrink)

David Albert claims that classical electromagnetic theory is not time reversal invariant. He acknowledges that all physics books say that it is, but claims they are ``simply wrong" because they rely on an incorrect account of how the time reversal operator acts on magnetic fields. On that account, electric fields are left intact by the operator, but magnetic fields are inverted. Albert sees no reason for the asymmetric treatment, and insists that neither field should be inverted. I argue, to the (...) contrary, that the inversion of magnetic fields makes good sense and is, in fact, forced by elementary geometric considerations. I also suggest a way of thinking about the time reversal invariance of classical electromagnetic theory -- one that makes use of the invariant (four-dimensional) formulation of the theory -- that makes no reference to magnetic fields at all. It is my hope that it will be of interest in its own right, Albert aside. It has the advantage that it allows for arbitrary curvature in the background spacetime structure, and is therefore suitable for the framework of general relativity. (The only assumption one needs is temporal orientability.). (shrink)