Epistemic extendedness, testimony, and the epistemology of instrument-based belief

In Relying on others [Goldberg, S. 2010a. Relying on others: An essay in epistemology. Oxford: Oxford University Press], I argued that, from the perspective of an interest in epistemic assessment, the testimonial belief-forming process should be regarded as interpersonally extended. At the same time, I explicitly rejected the extendedness model for beliefs formed through reliance on a mere mechanism, such as a clock. In this paper, I try to bolster my defense of this asymmetric treatment. I argue that a crucial assumption lying behind the argument I used to establish interpersonal extendedness in testimony cases does not apply to beliefs formed through reliance on instruments. In this respect, at least, there appears to be something epistemically distinctive about relying on another epistemic agent.