Knowledge is the attribute by which its object becomes clear to the knower in the relevant respect. The breadth of al-madhkūr allows intelligible judgments about existence, absence, and impossibility. Conception must be distinguished from judgment, and certainty from conjecture or merely intense conviction.
The three principal recognized routes for created knowers are sound senses, truthful report, and reason. Two questions must be kept separate: through which route is information acquired, and what is the ultimate theological account of its coming about? The distinctions between cause, instrument, and channel answer different aspects of these questions.
Observation supplies sensible data, reliable testimony supplies information beyond one's observation, and reason connects premises and consequences. An uncorroborated rumor about that same city, however, is not equivalent to established knowledge merely because it has been transmitted.