When we look at something, we perceive it in three dimensions. Our visual system allows us to perceive depth, which gives us a sense of the third dimension. This ability is primarily achieved through the process of stereopsis, where each eye views the object from a slightly different angle, providing slightly different perspectives. The brain combines these two different images to create a perception of depth.
In addition to stereopsis, other visual cues, such as perspective, shading, and motion parallax, also contribute to our perception of three-dimensional space. These cues help us judge distances, sizes, and positions of objects in relation to each other, further enhancing our perception of the three-dimensional world.
However, it's worth noting that the images projected onto our retinas are two-dimensional. The perception of depth and three-dimensionality is constructed by the brain using the visual information it receives from both eyes and various depth cues.