What do you know?

Wikipedia defines knowledge as a familiarity with someone or something, which can include facts, information, descriptions, or skills acquired through experience or education. It can refer to the theoretical or practical understanding of a subject.  The study of knowledge is called epistemology.

Plato famously defined knowledge as "justified true belief."  The problem with his construct is we must first know what belief is, what truth is and what constitutes justification. To believe something means any cognitive content held as true in spite of the absence of proof.

Belief

Example of an everyday belief:

A person believes that a particular bridge is safe enough to support him, and attempts to cross it; unfortunately, the bridge collapses under his weight. It could be said that he believed that the bridge was safe, but that this belief was mistaken. It would not be accurate to say that he knew that the bridge was safe, because plainly it was not. By contrast, if the bridge actually supported his weight then he might say he “thought” that the bridge was safe, and now after proving it to himself, he knows.

Why would the person think (believe) the bridge is safe? 

Perception:  sensory input makes the bridge appear safe – it is not rusty, it is not swaying in the breeze, the paving is not cracked, etc.; he sees others cross the bridge safely

Communication:  there are no signs warning of danger, e.g., high wind cautions; there have been no radio warnings about using the bridge; your friend who has recently used the bridge recommended you take it to get where you are going

Associations:  you have crossed many similar bridges in the past without incident; your experience is that things that tend to be unsafe show signs of potential danger, e.g., a car that appears poorly cared for is more likely to break down than one that is well cared for.

Experimentation:  you watch several people cross before you

Reasoning:  since all of the above are thought to be true, it is reasonable to believe the bridge is safe, but the man will not know that it is safe until he successfully crosses it.  Knowledge prior to any experience means that there are certain "assumptions" that one takes for granted, but if any of the assumptions are faulty, the conclusion may be faulty.

Truth

Truth is most often used to mean in accord with fact or reality. 

Theories of truth

Correspondence theories state that true beliefs and true statements correspond to the actual state of affairs.  This type of theory posits a relationship between thoughts or statements on one hand, and things or objects on the other.

Coherence theories require truth to properly fit elements within a whole system.

Social constructivism holds that truth is constructed by social processes, is historically and culturally specific, and that it is in part shaped through the power struggles within a community. Constructivism views all of our knowledge as "constructed," because it does not reflect any external "transcendent" realities (as a pure correspondence theory might hold). Rather, perceptions of truth are viewed as contingent on convention, human perception, and social experience. It is believed by constructivists that representations of physical and biological reality, including race, sexuality, and gender are socially constructed.

Consensus theory holds that truth is whatever is agreed upon, or in some versions, might come to be agreed upon, by some specified group.

Pragmatic theory essentially states that what works may or may not be true, but what fails cannot be true because the truth always works.

Criteria of truth

Authority, Revelation

Coherence

Consensus, Majority rule:  such as the laws of logic and mathematics.

Consistency:  if all A's are B's and all B's are C's, then all A's are C's.

Custom, Tradition

Intuition, Instinct, Emotions

Pragmatic

Justification

In order to know that a given proposition is true, one must not only believe the relevant true proposition, but one must also have a good reason for doing so. One implication of this would be that no one would gain knowledge just by believing something that happened to be true. For example, an ill person with no medical training, but with a generally optimistic attitude, might believe that he will recover from his illness quickly. Nevertheless, even if this belief turned out to be true, the patient would not have known that he would get well since his belief lacked justification.

Things you know:

Knowledge that, knowledge how, and knowledge by acquaintance:  it is known that 2 + 2 = 4, but there is also knowing how to add two numbers and knowing a person (e.g., oneself), place (e.g., one's hometown), thing (e.g., cars), or activity (e.g., addition).

I know that water freezes at 32^o F.

I know if I drop a ball it will fall to the ground.

I know how to use PowerPoint

I know how to get home from here

I know Alice because I have met her before.

I know John can be trusted to get here on time.