One of the problems with defining the word knowledge is that it needs to include a very broad scope of ideas. Higher levels of it are tacit. The challenge here is not only to convey what it is, but to create a vocabulary that can explain it and be universally accepted.
What makes justified belief justified and how do you know that you know? In philosophy, epistemology is the study that tries to understand what knowledge is. It’s basis, form, nature, validity and limits are widely debated. One of the things epistemologists do is to investigate the justifications supporting various beliefs of knowledge and then try to trace them to their origin. There are many different theories of knowledge and methods used to cross reference the findings, in hope of distinguishing true claims from false ones.
Epistemological knowledge foundations and structures are built from two opposing beliefs, empirical and rational. Empiricism is the view that experience and observation should be the means of gaining knowledge, a posteriori knowledge. Rationalism is the standpoint that knowledge can be discovered through pure reason and thought, a priori knowledge.
Skepticism is another issue and study in epistemology to deal with, because knowledge believed by one person isn’t necessarily believed by another, even when it’s widely excepted. There’s a whole spectrum of skepticism ranging from people that can eventually be convinced, to extreme skepticism that no matter how much evidence is presented, they won’t believe it. Skepticism has also been used constructively in epistemology.
Philosophical skepticism questions the possibility of certainty in knowledge. Methodological skepticism systematically scrutinizes knowledge claims by sorting out true from false claims. One form of methodological skepticism is Cartesian doubt, named after Rene Descartes. This is to doubt the truth of all beliefs at first, in order to determine which ones are true. This is the approach Descartes used to derive his statement, Cogito ergo sum. This translates to I think, therefore I am.
In spite of the many different opinions in epistemology, it still helps form strong ideas of what knowledge is. When answering the big questions, an epistemic criteria for what constitutes knowledge will be a prerequisite.