The cognitive barrier in understanding or proving the existence of spatial dimensions beyond our 3D perception. We can describe them mathematically, but cannot visualize or directly experience them. Any purported evidence of higher dimensions (like unexplained forces or entities) is necessarily filtered through our 3D senses and instruments, meaning it will always appear as a mysterious anomaly within our known physics, not as clear proof of "other dimensions." The gap between the math and the manifest experience is unbridgeable.
*Example: "The mystic said he perceived 4D shapes during meditation. The hard problem of N-Dimensionality: any description he gives will be a 3D shadow or metaphor ('it was like a constantly turning inside-out cube'). We have no language or sensorium for the real thing. Proof is impossible, because any evidence would, by definition, manifest within our dimensional prison."*
A tough, straightforward, take-no-prisoners way of carrying oneself. Hard-nosed people may not be sensitive or tactful, but they are gritty and tackle problems head-on.
Person 1: Heard about some guy named Collin Klein today, apparently he's in the Heisman race..?
Person 2: Yeah bro, he's the quarterback for K-state. That kid plays old school hard-nosed football.
OR
Marine 1: You hear about the gunny sarge? He took a bullet to the leg and didn't even notice. Wiped out the whole sniper nest by himself.