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If we see the distant objects with one eye, they will appear as clear as they would by both eyes. A difference occurs, if we see comparatively nearer objects. If we close our left eye, we cannot see the objects of left side. And if we close the right eye, we cannot see the objects of right side. With the help of both eyes we can get the correct assessment of distance. We can prove it by the following experiment. In your one hand take a pin and in the other hand take a comb. Now try to place the tip of the pin on one of the teeth of the comb. It becomes easy to do if both the eyes are open. But it takes more time to bring one point on the other if one of the eyes is closed. Only two dimensions of an object are perceived by us if we see the object with one eye. But if we see the object with both eyes, we see the objects in their totality. We see the objects of left side with left eye and the objects of right side with right eye. Therefore both the eyes make possible to have complete knowledge of the solidity and depth of the object. Another advantage of having two eyes is that a person does not become totally blind if one of the eyes of a person gets damaged because of some reason.
We also have only two eyes because we, unlike most other creatures, are able to turn our heads in multiple directions without completely turning our bodies. This allows us to only have need of two eyes. Some creatures, like spiders for example, don't have a swivel/pivot neck. So they need eight eyes placed on different parts of their head to allow them to see what they would otherwise need to turn completely around to see.