The human eye has a blind spot in its field of vision. This lies on the point of the retina where the optic nerve leads back into the brain. The retina has no light-sensitive rods or cones at this point, and so a small object in the field of vision's blind spot becomes invisible.
The diagram shows how the optic nerves for left and right eyes are arranged symmetrically, so that the blind spot of the right eye lies somewhere right of the centre of vision, and the blind spot of the left eye lies somewhere off to the left of centre. Since the right eye can see whatever lies in the left eye's blind spot, and vice versa, the two eyes together provide complete vision.
You might think it's easy to find these blind spots: just close one eye and look for a grey fuzzy area. But there isn't any grey fuzzy area. Strange.
In order to find the blind spot of the right eye, it is necessary to close the left eye, focus the right eye on a single point, and see if anything vanishes from vision some 20 degrees right of this point. The following diagram has a set of characters on the left hand side, and black circle on the right. Keeping your head motionless, with the right eye about 3 or 4 times as far from the page as the length of the red line, look at each character in turn, until the black circle vanishes.
With increasing age, the blind spot enlarges. You may find that the black circle disappears when several of the characters are looked at. The size and shape of the blind spot can be found if a large enough grid of characters is used.
The same test can be done for the left eye. Close the right eye, and look at each character until the black circle disappears.
Note that when the black circle vanishes, you see only a white background where the circle was. What happens if the background colour is different? Say, green.
The blind spot appears as green. This is interesting, because it means that, although my eye can't detect anything in the blind spot, something knows that it is surrounded by green, and has guessed that what is in the blind spot is probably green. Smart!
How smart? If a thick horizontal line is drawn through the blind spot, what happens then?
The answer, it seems, is that if the line passess right through the blind spot, whatever is making shrewd guesses about colours is also able to work out that a line going in one side and coming out the other probably continues through the middle. The black circle disappears, but the line remains.
So what happens when a pen or pencil is pushed into the blind spot? It seems that as the tip enters the blind spot, the pencil appears truncated, as if it were vanishing into something (which, after all, it is). But when the tip emerges at the other side, the visual processing system fills in the missing part between. The following animation mimics pushing a pencil into the blind spot.
The first conclusion drawn from this little experiment is that, although each eye has a blind spot, some sort of intelligence is used to give this area not only a likely colour, but also to fill in lines that pass through the blind spot - rather than just have a fuzzy grey area. The net result is that, with one eye closed, it isn't immediately obvious where the blind spot is, because it has been given a suitable colour, and even pattern, based on what is adjacent to it.
The second conclusion drawn is that what we see is not just what has appeared on the retina, but is an image that has been reprocessed, tidied up. And if the human visual cortex is able to tidy up the blind spot, then it may well be that the same is being done for the entire visual field - that what we get to 'see' is not what appears on the retina, like a photograph, but instead something which has a whole bunch of special effects added.
If so, then we can't trust our eyes. We're being given doctored information, massaged figures. The world that we see is not something out there, but a world that we invent. The world I see is my idea.