Bohr's response was that the wall is quantum mechanical as well, and that to measure the recoil to accuracy Δ p, the momentum of the wall must be known to this accuracy before the particle passes through. This introduces an uncertainty in the position of the wall and therefore the position of the slit equal to ⁠ h / Δ p ⁠, and if the wall's momentum is known precisely enough to measure the recoil, the slit's position is uncertain enough to disallow a position measurement.