In 1869, after Beltrami 's letter... he realized he had made a mistake: the empirical concept of a rigid body and mathematics alone were not enough to characterize Euclidean geometry.
Sa 1869, matapos Beltrami 's titik... siya na maisasakatuparan niya na ginawa ng isang pagkakamali: ang konsepto mula sa obserbasyon ng isang naninigas body at matematika nag-iisa ay hindi sapat upang makilala Euclidean geometry.