The assumption that knowledge is best understood as possessed by individuals (as discussed last week) fits some sources of knowledge very well. Knowledge underpinned by perception, reasoning, whether inductive or deductive, and memory seems to be a matter of an individual perceiving, reasoning or remembering. But there is another source of knowledge: testimony which in philosophy (by contrast with the law) just means acquiring knowledge from others.
So if individualism about knowledge is right then it seems that testimony needs to be justified by an individual for him- or herself. Hume seems to think this when he says:
[Our trust in testimony derives from] no other principle than our observation of the veracity of human testimony, and of the usual conformity of facts to the reports of witnesses. It being a general maxim, that no objects have any discoverable connexion together, and that all the inferences, which we can draw from one to another, are founded merely on our experience of their constant and regular conjunction; it is evident, that we ought not to make an exception to this maxim in favour of human testimony, whose connexion with any event seems, in itself, as little necessary as any other... And as the evidence, derived from witnesses and human testimony, is founded on experience so it varies with the experience and is regarded either as a proof or a probability according as the conjunction between any particular kind of report and any kind of object has been found to be constant or variable. Hume, D. ([1748] 1975) Enquiries Concerning Human Understanding and Concerning the Principles of Morals, Oxford: Clarendon Press §88, 111-2.
But as a number of recent philosophers have argued, no such individualistic justification of testimony is possible. And if it is not, then it seems that at least some forms of knowledge are essentially social.
Kusch, M. (2002) Knowledge by Agreement, Oxford: Oxford University Press pp29-36.
Other texts
Coady, C. (1973) ‘Testimony and Observation’ American Philosophical Quarterly, 10: 149-155 (on WebCT)
Fricker, E. (1987) 'The epistemology of testimony' Proceedings of the Aristotelian Society, Supplementary Volumes, 61: 57-81 (link to JStor)
Lipton, P. (1998) 'The epistemology of testimony’ Studies in History and Philosophy of Science 29: 1–31. (link to external site)
The slides are here.