l" reason to examine a posteriori knowledge. Bertrand Russell, writing in the modern period, distinguishes between knowledge by description, roughly equivalent to a priori or "propositional" knowledge, and knowledge by acquaintance, roughly equivalent to a posteriori or nonpropositional knowledge (Moser & Nat, 1995, 3-4). Russell develops the idea that there is a logical, implicit relation between a sentence that is a definite description of an object and a statement of the existence of that object, and knowledge thereof is accomplished by acquaintance with (not direct experience of) the "constituents" or particulars, of that object (Russell, 1995, p. 222). Even a description that is not an object of direct acquaintance (Russell gives the example of Julius Caesar) may be an object because certain attributes are associated with that description. Thus about such descriptions/objects, knowledge can be accomplished.
Now a belief may be accurate knowledge or not; angels exist or do not. That seems a relatively simple conclusion to make since the evidence for angels is not tangible. But that does not mean that a posteriori/nonpropositional/practical knowledge is always accurate. That is because the senses cannot always be trusted either: Water seems but is not (necessarily) blue; the Ptolemaic universe seems but is not (as Copernicus and Galileo demonstrated) an adequate account of the cosmos. Because verifying knowledge is problematic, the method used to arrive at knowledge assumes importance.
Logic is one method by which the truth is arrived at. Logic involves rational processes, and it can be applied to both purely mental activity and mental plus physical/empirical activity. Aristotle formulated a system of logic as a method of reason and argument. However, Aristotle did not dispose once and for all of the connection between logic and knowledge, since questions of epistemology persist. Philosophers continually address how we know wh...