what is knowledge? Sources of knowledge?....

Knowledge is a term with many meanings depending on context, but is as a rule closely related to such concepts as information, communication, meaning, instruction, representation, learning and mental stimulus. The study of knowledge is called epistemology. In epistemology, a common definition of knowledge is that it consists of justified true belief.

Knowledge includes, but is not limited to, those descriptions, hypotheses, concepts, theories, principles and procedures which to a reasonable degree of certainty are either true or useful.

Knowledge consists of beliefs about reality. One way of deriving and verifying knowledge is from tradition or from generally recognized authorities of the past, such as Aristotle. Knowledge may also be based upon the pronouncements of secular or religious authority such as the state or the church. A second way to derive knowledge is by observation and experiment: the scientific method. Knowledge may also be derived by reason from either traditional, authoritative, or scientific sources or a combination of them and may or may not be verified by resort to observation and testing.

Knowledge may be factual or inferential. Factual knowledge is based on direct observation. It is still not free of uncertainty, as errors of observation or interpretation may occur, and any sense can be deceived by illusions. Inferential knowledge is based on reasoning from facts or from other inferential knowledge such as a theory. Such knowledge may or may not be verifiable by observation or testing. For example, all knowledge of the atom is inferential knowledge. The distinction between factual knowledge and inferential knowledge has been explored by the discipline of general semantics.

Roger Bacon, an English alchemist and philosopher of the high middle ages, had this to say about knowledge: "Of the three ways in which men think that they acquire knowledge of things - authority, reasoning, and experience - only the last is effective and able to bring peace to the intellect." Thus knowledge might arise from authority, logic, or experience. Earlier Divine illumination by the grace of God was contrasted by the early Christian church with knowledge gained by reason such as practiced by classical philosophers such as Plato and Aristotle. Experimental knowledge was discounted,

Sources of knowledge :

Belief
Memory
Perception

Belief:

Belief is the mental acceptance of and conviction in the validity of an idea, usually based on some combination of faith, reason and, in some cases, experience.

memory:

In psychology, memory is an organism's mental ability to store, retain and recall information. Traditional studies of memory began in the fields of philosophy, including techniques of artificially enhancing the memory. The late nineteenth and early twentieth century put memory within the paradigms of cognitive psychology. In recent decades, it has become one of the principal pillars of a branch of science called cognitive neuroscience, an interdisciplinary link between cognitive psychology and neuroscience.

Perception:

In philosophy, psychology, and the cognitive sciences, perception is the process of attaining awareness or understanding of sensory information. It is a task far more complex than was imagined in the 1950s and 1960s, when it was predicted that building perceiving machines would take about a decade, a goal which is still very far from fruition. The word comes from the Latin words perceptio, percipio, and means "receiving, collecting, action of taking possession, apprehension with the mind or senses"

Perception is one of the oldest fields in psychology. The oldest quantitative law in psychology is the Weber-Fechner law, which quantifies the relationship between the intensity of physical stimuli and their perceptual effects. The study of perception gave rise to the Gestalt school of psychology, with its emphasis on holistic approach.

What one perceives is a result of interplays between past experiences, including one’s culture, and the interpretation of the perceived. If the percept does not have support in any of these perceptual bases it is unlikely to rise above perceptual threshold.