Though influence from the west is generally accepted, especially the Persian columns of Achaemenid Persia, there are a number of differences between these and the pillars. Persian columns are built in segments whereas Ashokan pillars are monoliths, like some much later Roman columns. Most of the Persian pillars have a fluted shaft while the Mauryan pillars are smooth, and Persian pillars serve as supporting structures whereas Ashokan pillars are individual free-standing monuments. There are also other differences in the decoration. Indian historian Upinder Singh comments on some of the differences and similarities, writing that "If the Ashokan pillars cannot in their entirety be attributed to Persian influence, they must have had an undocumented prehistory within the subcontinent, perhaps a tradition of wooden carving. But the transition from stone to wood was made in one magnificent leap, no doubt spurred by the imperial tastes and ambitions of the Maurya emperors."