Plato's Allegory of the Cave illustrates well the predicament we find ourselves in.

 

Here is a synopisis from Wikipedia:

Imprisonment in the Cave

Socrates begins by asking Glaucon to imagine a cave where people have been imprisoned from childhood. These prisoners are chained so that their legs and necks are fixed, forcing them to gaze at the wall in front of them and not look around at the cave, each other, or themselves (514a–b). Behind the prisoners is a fire, and between the fire and the prisoners is a raised walkway with a low wall, behind which people walk carrying objects or puppets "of men and other living things" (514b). The people walk behind the wall so their bodies do not cast shadows for the prisoners to see, but the objects they carry do ("just as puppet showmen have screens in front of them at which they work their puppets" (514a)). The prisoners cannot see any of this behind them and are only able to see the shadows cast upon the cave wall in front of them. The sounds of the people talking echo off the shadowed wall, and the prisoners falsely believe these sounds come from the shadows (514c).

Socrates suggests that the shadows constitute reality for the prisoners because they have never seen anything else; they do not realize that what they see are shadows of objects in front of a fire, much less that these objects are inspired by real living things outside the cave (514b-515a).

Departure from the Cave

Allegory of the Cave. Left (From top to bottom): Sun; Natural things; Shadows of natural things; Fire; Artificial objects; Shadows of artificial objects; Allegory level.
Right (From top to bottom): "Good" idea, Ideas, Mathematical objects, Light, Creatures and Objects, Image, Analogy of the Sun, and the Analogy of the Divided Line

Socrates then supposes that one prisoner is freed, being forced to turn and see the fire. The light would hurt his eyes and make it hard for him to see the objects that are casting the shadows. If he is told that what he saw before was not real but that the objects he is now struggling to see are, he would not believe it. In his pain, Socrates continues, the freed prisoner would turn away and run back to what he can see and is accustomed to, that is the shadows of the carried objects. He writes "...it would hurt his eyes, and he would escape by turning away to the things which he was able to look at, and these he would believe to be clearer than what was being shown to him."

Socrates continues: "suppose...that someone should drag him...by force, up the rough ascent, the steep way up, and never stop until he could drag him out into the light of the sun."  The prisoner would be angry and in pain, and this would only worsen when the radiant light of the sun overwhelms his eyes and blinds him. The sunlight is representative of the new reality and knowledge that the freed prisoner is experiencing.

Slowly, his eyes adjust to the light of the sun. First he can only see shadows. Gradually he can see the reflections of people and things in water and then later see the people and things themselves. Eventually he is able to look at the stars and moon at night until finally he can look upon the sun itself (516a). Only after he can look straight at the sun "is he able to reason about it" and what it is (516b). (See also Plato's Analogy of the Sun, which occurs near the end of The Republic, Book VI.)

Return to the Cave

Socrates continues, saying that the freed prisoner would think that the real world was superior to the world he experienced in the cave; "he would bless himself for the change, and pity [the other prisoners]" and would want to bring his fellow cave dwellers out of the cave and into the sunlight (516c).

The returning prisoner, whose eyes have become acclimated to the light of the sun, would be blind when he re-enters the cave, just as he was when he was first exposed to the sun (516e). The prisoners, according to Socrates, would infer from the returning man's blindness that the journey out of the cave had harmed him and that they should not undertake a similar journey. Socrates concludes that the prisoners, if they were able, would therefore reach out and kill anyone who attempted to drag them out of the cave (517a).

 

 

Here is the actual text. I have not included, and do not agree with Plato's reasoning and application beyond his actual allegory.

[Socrates] And now, I said, let me show in a figure how far our nature is enlightened or unenlightened: --Behold! human beings living in a underground cave, which has a mouth open towards the light and reaching all along the cave; here they have been from their childhood, and have their legs and necks chained so that they cannot move, and can only see before them, being prevented by the chains from turning round their heads. Above and behind them a fire is blazing at a distance, and between the fire and the prisoners there is a raised way; and you will see, if you look, a low wall built along the way, like the screen which marionette players have in front of them, over which they show the puppets.

[Glaucon] I see.

[Socrates] And do you see, I said, men passing along the wall carrying all sorts of vessels, and statues and figures of animals made of wood and stone and various materials, which appear over the wall? Some of them are talking, others silent.

[Glaucon] You have shown me a strange image, and they are strange prisoners.

[Socrates] Like ourselves, I replied; and they see only their own shadows, or the shadows of one another, which the fire throws on the opposite wall of the cave?

[Glaucon] True, he said; how could they see anything but the shadows if they were never allowed to move their heads?

[Socrates] And of the objects which are being carried in like manner they would only see the shadows?

[Glaucon] Yes, he said.

[Socrates] And if they were able to converse with one another, would they not suppose that they were naming what was actually before them?

[Glaucon] Very true.

[Socrates] And suppose further that the prison had an echo which came from the other side, would they not be sure to fancy when one of the passers-by spoke that the voice which they heard came from the passing shadow?

[Glaucon] No question, he replied.

[Socrates] To them, I said, the truth would be literally nothing but the shadows of the images.

[Glaucon] That is certain.

[Socrates] And now look again, and see what will naturally follow if the prisoners are released and disabused of their error. At first, when any of them is liberated and compelled suddenly to stand up and turn his neck round and walk and look towards the light, he will suffer sharp pains; the glare will distress him, and he will be unable to see the realities of which in his former state he had seen the shadows; and then conceive some one saying to him, that what he saw before was an illusion, but that now, when he is approaching nearer to being and his eye is turned towards more real existence, he has a clearer vision, -what will be his reply? And you may further imagine that his instructor is pointing to the objects as they pass and requiring him to name them, -will he not be perplexed? Will he not fancy that the shadows which he formerly saw are truer than the objects which are now shown to him?

[Glaucon] Far truer.

[Socrates] And if he is compelled to look straight at the light, will he not have a pain in his eyes which will make him turn away to take and take in the objects of vision which he can see, and which he will conceive to be in reality clearer than the things which are now being shown to him?

[Glaucon] True, he now.

[Socrates] And suppose once more, that he is reluctantly dragged up a steep and rugged ascent, and held fast until he 's forced into the presence of the sun himself, is he not likely to be pained and irritated? When he approaches the light his eyes will be dazzled, and he will not be able to see anything at all of what are now called realities.

[Glaucon] Not all in a moment, he said.

[Socrates] He will require to grow accustomed to the sight of the upper world. And first he will see the shadows best, next the reflections of men and other objects in the water, and then the objects themselves; then he will gaze upon the light of the moon and the stars and the spangled heaven; and he will see the sky and the stars by night better than the sun or the light of the sun by day?

[Glaucon] Certainly.

[Socrates] Last of he will be able to see the sun, and not mere reflections of him in the water, but he will see him in his own proper place, and not in another; and he will contemplate him as he is.

[Glaucon] Certainly.

[Socrates] He will then proceed to argue that this is he who gives the season and the years, and is the guardian of all that is in the visible world, and in a certain way the cause of all things which he and his fellows have been accustomed to behold?

[Glaucon] Clearly, he said, he would first see the sun and then reason about him.

[Socrates] And when he remembered his old habitation, and the wisdom of the cave and his fellow-prisoners, do you not suppose that he would felicitate himself on the change, and pity them?

[Glaucon] Certainly, he would.

[Socrates] And if they were in the habit of conferring honors among themselves on those who were quickest to observe the passing shadows and to remark which of them went before, and which followed after, and which were together; and who were therefore best able to draw conclusions as to the future, do you think that he would care for such honors and glories, or envy the possessors of them? Would he not say with Homer, "Better to be the poor servant of a poor master," and to endure anything, rather than think as they do and live after their manner?

[Glaucon] Yes, he said, I think that he would rather suffer anything than entertain these false notions and live in this miserable manner.

[Socrates] Imagine once more, I said, such an one coming suddenly out of the sun to be replaced in his old situation; would he not be certain to have his eyes full of darkness?

[Glaucon] To be sure, he said.

[Socrates] And if there were a contest, and he had to compete in measuring the shadows with the prisoners who had never moved out of the cave, while his sight was still weak, and before his eyes had become steady (and the time which would be needed to acquire this new habit of sight might be very considerable) would he not be ridiculous? Men would say of him that up he went and down he came without his eyes; and that it was better not even to think of ascending; and if any one tried to loose another and lead him up to the light, let them only catch the offender, and they would put him to death.

[Glaucon] No question, he said.