Adding to the fun of black-ravens-as-knowledge-metaphor is Immanuel Kant's concept of the analytic-synthetic dichotomy.

Kant proposed that all such assertions can be categorized as either "analytic", in which the entity in question ("ravens") have an attribute by definition ("blackness"), or "synthetic", in which the attribute is not a defining characteristic of the entity is question, but rather added to the basic concept.

The issue becomes, in the case of making an assertion such as "All ravens are black", Kant would argue, that we are fundamentally just arguing about definitions. That is, either "ravens" are defined as being "black" birds (analytic), or "blackness" is an observed, non-defining characteristic of "ravens" (synthetic). Any such assertion comes down to how you (arbitrarily) define the entity in question. In other words, Kant would likely throw the question back and ask, "What do you mean by 'raven', and why is that definition valid?"

This dichotomy, though it potentially invalidates all human concepts, remains tenuously answered. The precise, "correct" definition of any given concept is approached through using Occam's Razor, but that method relies on a very debatable use of the criteria of "necessity".

(And I came to this node because I thought it was about Huginn and Muninn... "Thought" and "Memory"... hmm...)