Through some years of philosophical study I've become confused about what exactly it means for me to have knowledge. What was once a familiar and seemingly clear concept has now become unfamiliar and obscure. Can it be made clear again for me? Can I ever know whether or not I know? It seems as though the more I read about knowledge the more obscured it becomes.

The topic of knowledge is an old one, going back at least to Plato who wrestled with the difference between knowledge and correct opinion. The traditional, most common understanding of knowledge is that a person knows X (whatever X stands for) if that person has a true, justified belief about X. Justification refers to evidence. This traditional understanding of knowledge has been challenged on the grounds that you might have a justified true belief about X --that Pat Jones is in Spain-- and yet the justification / evidence is spurious, e.g. imagine you are seeing Jones' identical twin, Chris Jones, in Spain and you inappropriately conclude you are actually seeing Pat. This has caused some philosophers to amend the definition to:

A person knows X if the person's belief is true and the evidence for this belief does not involve essential reasoning by way of a false premise.

Matters that remain unsettled include (a) Just how much evidence or justification is needed for one's belief to count as knowledge, and not simply a reasonable assumption? Some philosophers press for a strict, unsurpassable amount of evidence, to the effect that knowledge claims need to be infallible (not subject to error) and incorrigible (not subject to change); others adopt more flexible, less strict accounts (b) Can you know X and not be able to reconstruct the evidence that your belief(s) about X are based upon? There is a view, sometimes called reliabilism, according to which a person may know X if her beliefs are reliably true, even when the person is not consciously aware of the relevant evidence. (c) Can you know x without knowing that you know X? Many philosophers think this is possible. Children and some nonhuman animals may have knowledge without knowing that they have such knowledge. (d) Is there a significant difference between the forms of knowledge we express by the phrases "knowing how" and "knowing that"? Some philosophers contend that someone might know how to swim or ride a bike without being able to put such knowledge-claims in the form of propositions.

I believe that there is some variation in the English usage of "knowledge." I have heard people say "I knew that _______ but then I realized I was mistaken." On the traditional view, if you actually know that (say) Jones is not in Spain, it cannot be the case that Jones is in Spain.

Good luck in your philosophy! The topic of knowledge (its nature and value) is relevant to almost every domain of philosophy.

I don't know the answer to your question, but since this topic interests you, I would recommend you take a look at the skeptical traditions generally categorized as Pyrrhonian and Academic skepticism. One famous device you might use to think about these questions is called Agrippa's trilemma. An ancient chronicler of skepticism called Sextus Empiricus reports that one Agrippa posed the following problem: Justifications for knowledge claims seem problematic because knowledge claims must be justified by other claims, just as premises are needed to justify a conclusion. How are the justifying claims to be themselves justified? Either (1) they are self-evident and self-justifying--but this seems wrong and little better than making assumptions, which justify nothing. Or (2) the supposed justification starts an infinite regress where the supporting claims get justified by other claims and those claims get justified by still other claims, ad infinitum--but an infinite regress doesn't seem like justification. Or (3) the justifying claims run finally in a circle where one set justifies others, which are justified by others, which require the initial claim for justification--but arguing in a circle doesn't justify anything either. Since there's no alternative besides stopping with assumptions, an infinite regress of justifications, or arguing in a circle, and since none of those three offer adequate justification, then it seems that knowledge is impossible. For Sextus's formulation of this trilemma, see his book, _The Outlines of Pyrrhonism_, Book 1, Chapter 15, Line 164 (usually cited as PH 1.15.164). Do you see a way out of Agrippa's trilemma? I confess that as of this moment, I don't.

Read another response by Charles Taliaferro, Peter S. Fosl
Read another response about Knowledge