spacetime dimensions - Perception of Depths and Stereovision

I watched a video on Youtube and read a few articles which say that human beings see 3D because we have two eyes. But does that mean when I close one of my eyes, I should see 2D? It doesn't happen so. Why am i still able to be understand depth? Would a person with one eye not be able to perceive depths?

Answer

You can't understand depth as well with one eyed sight. But the "photogrammetry" (the process of inferring depth from images) done by our minds only partly depends on stereopsis (depth inference from images on taken at slightly different positions / in planes at slight angles). The mind subconsciously brings to bear all kinds of heuristics and rules it has learnt when processing images. For example, we know from experience roughly how big certain objects are: if we see a car in the distance, the mind knows that it is roughly 2 meters wide and four to five meters in length; an inference of distance to the object is clearly made from this information. The very fact that our minds do this and we can be consciously aware of these calculations if prompted in the right way is the reason behind the humor of this classic Gary Larson cartoon:

On the African Savannahs where we evolved our sight, there are many "standard" lengths the mind can use like this to infer distance: trees and plants, well kenned landmarks and so on. Indeed just as the parallax method of measuring distances to stars (measuring of the angle subtended by 1AU through images taken six months apart) becomes less accurate with distance, gives out at about 300 light years distance and must be replaced by standard candles / redshift measurements, so too does our mind gives less weight to subconscious stereoscopic calculations and more weight to heuristics like the above. It would be an interesting psychological experiment to test the accuracy of depth perception of one eyed versus two eyed sight with distance: the advantage of two eyes over one would lessen with distance. I'm sure something like this has been done, but cannot find references to it right now.