 Investigations of the parallel postulate began in Greek times, continued in the Islamic world, and were undertaken in the early modern West. But, for reasons which are still unclear, after around 1800 it became easier for people to imagine that Euclid’s Elements might not be the only possible system of metrical geometry. Among the factors that can help explain how the unthinkable became thinkable even outside the community of mathematicians was the accumulation of theorems based on assumptions other than the parallel postulate. It would seem that the production of novel, consistent consequences of such a radical assumption, and the failure to find a contradiction, inclined some people to contemplate that there might indeed be a whole geometry different from Euclid’s. It may also be relevant that this process coincided with the full assimilation of imaginary numbers (and the rise of complex function theory); indeed, Lobachevskii called his theory "Imaginary geometry".