Right: We're only terrible at perceiving constant velocity without visual reference because it's physically impossible to do so in a controlled environment, acc

Right: We're only terrible at perceiving constant velocity without visual reference because it's physically impossible to do so in a controlled environment, according to Newton. What we're terrible at is perceiving constant acceleration in very fine increments, like 10 milli-G while standing, or on up past 100 milli-G while sitting or prone. This is directly equivalent to sensing a certain slope/grade in the terrain, if one is robbed of accurate horizontal (horizon) and vertical (trees/buildings) reference. The mind is capable of tolerating several degrees of tilt while being perfectly convinced everything is flat, so long as the visual references point in that direction... and even at greater extremes we really only notice topography when it's highly variable, cliffs and abrupt hills and sharp changes in slope. I have an unconfirmed notion that our ability to, for example, carry things on our back, or walk while pregnant, would be sharply curtailed if our body didn't automatically adjust to the different center of mass, and different perceived gravity vector from the standpoint of our skeletal centerline. Humans directly perceive jerks, and they perceive them with alarm, because if the ground is jerking in a natural context it means you're about to fall off a slope and die.