An Analogy. Mere opinion is to knowledge as stealing is to honest toil.
Now, "true opinion" is somewhere in between. "Merely true opinion" is closer to "mere opinion" (on a spectrum) than it is to "knowledge." And "justified true opinion [belief]" is closer to "knowledge" (on the spectrum). So we have the following:
This is very much like there is a continuum between mere thievery and honest toil. Of course someone would argue against both counts believing that there is a precise point at which an action becomes stealing: there are necessary and sufficient conditions for stealing, and these conditions (taken together) are logically exclusive to the necessary and sufficient conditions of honest toil. And they would say the same of the relationship between mere opinion and knowledge.
But Let me illuminate further: acquisition of knowledge requires a certain type of experience that acquisition of mere opinion does not.
Socratic moment: How can I speak about knowledge when I do not as yet have a definition of it? Well, first of all I am not trying to come up with a definition [hence I do not assume the primacy of definition], and second, I am able to use the term knowledge coherently in a community of speakers [unless I'm actually crazy or something] hence I am legitimate in speaking about knowledge; for I am speaking about he use of the term 'knowledge' and the proper use gives way to a proper understanding of the term which comprises the proper meaning.
What is the nature of this "certain type of experience"? I have already spoken of the analogy: knowledge is like honest toil. If I seek knowledge of calculus I will read about it, trying to understand what someone has written about it; I will work problems, and someone will check them. When I make mistakes, I will strive to correct them. Hence I come to have knowledge about calculus. There is not one essence to this experience but a family resemblance (to use an increasingly hackneyed term) amongst all of these experiences.
How does one know a person? You spend time with that person--in discussion, in silence, discovering what makes that particular person an individual.
How does one know virtue? One experiences the actions of others who are called "virtuous" and one mimics those individuals.
So we see that knowledge and experience are closely related. Having knowledge is not the satisfaction of some necessary and sufficient conditions, but is the gaining of experience with respect to the thing to be known.
