Humans don’t really know 3D. Only a partly wrapped-around surface.
(This has consequences for how we think and measure which I’ll do in another post)
Example: Eyes and Hands.
Vision: We don’t see 3D. People who can see ‘3D’ (I can’t) see a “wrap-around” view of a focal object.
A little left set forward or back, a little right set forward or back, and a middle set forward or back when it is within Panum’s area of fusion, otherwise double vision.
If you have stereoscopic vision, the object seems to “pop” forward. As you scan, other objects can also “pop” forward or back.
But this wrap-around view does not see the back of objects and it only sees the surfaces, not the insides. (see Horopter)
b) Hands / any body touch of objects.
Hands wrap around 3D objects, measuring the surface characteristic as far as we can wrap around them. Mapping Pressure.
We know surfaces, weight, shape and size. Our hands wrap around. We can squish some objects for density or use other means such as tapping and listening to the sound.
If we put our hands or body inside of an object or space, we can estimate density (water vs air) and volume to some extent by touch.
But these are all surface measurements, either the measure of an external surface, or the measure of an external environment upon our own surface (physically measuring density for example).
Thankfully we can combine these surfaces, masses, shapes and volumes in various ways to come up with some kind of cohesive object-ness.
But it’s still a wraparound. The label on a can of green beans. That’s what our vision and touch can do. We don’t know there’s green beans inside as we can’t see or touch in 3D unless there is a 2D surface of some kind (curved, flat, rippled, porous but still a 2D surface with which to measure with). Not 3D but various 2D approximations of 3D combined).
This goes for you too hearing. It’s still amazing of course.
“The Merkel cells — small receptor cells clustered at the centre of the fingerprint ridge and in small domes elsewhere on the body — signal the weight, form, and surface features of objects contacting the skin with a continuous, slowly-adapting spike train proportional to pressure. “