Is there a single word for someone who forms their own opinion based entirely on their personal experience, without having been influenced by any outside source?
I'd probably use:
A related word is autogenic, an adjective indicating something is self-produced. One might apply the term autogenicist to a person who has been informed solely by his or her own experience.
Whatever name you give the self-informed person, autodidact (“A self-taught person; an automath”) is a hypernym of that name. That is, the set of self-informed persons is a subset of the set of autodidacts.
Two previously-suggested terms are useful in spite of being ignored or voted down. Empiricism (“A pursuit of knowledge purely through experience, especially by means of observation and sometimes by experimentation”) is directly relevant, and leads to the suggestion empiricist. Solipsism (“The theory that the self is all that exists or that can be proven to exist” or “Self-absorption, an unawareness of the views or needs of others”) is slightly less relevant, but for someone to be completely self-informed, it is probable that person must be solipsistic or a hermit or deranged or autistic or perhaps a genius.
In some contexts you can use:
Possibly the most controversial feature of the solipsistic worldview is the denial of the existence of other minds. Since personal experiences are private and ineffable, another being's experience can be known only by analogy.
A man that forms his own opinion should be open to hear opinion of the others and then to make his own. How a man can make his opinion without been influenced by any outside source as he has his social and education "environment" by default ? Your question is not exactly clear about that.
My suggestion is open-minded man.
Intuition has the following definitions:
a. an immediate cognition of an object not inferred or determined by a previous cognition of the same object.
b. any object or truth so discerned.
c. pure, untaught, noninferential knowledge. (emphasis mine)
Unadulterated : (Source)
not mixed or diluted with any different or extra elements; complete and absolute
"He is of an unadulterated mind."
Independent : (Source)
- Not governed by a foreign power; self-governing.
- Free from the influence, guidance, or control of another or others; self-reliant
- Not determined or influenced by someone or something else; not contingent
"He considers himself an independent, having formed his own thoughts and views without any whispers in his ear."
The above is for your request: "Is there a single word[...]?"
(edits = mine)