Any sufficiently advanced magic is indistinguishable from technology – Niven’s Corollary to Clarke’s Law
Elves & Espers is a setting for D&D-like games, taking place in the far (retro) future of the past of typical D&D worlds. It’s a future of rocket ships, ray guns, psionic powers…all powered by magic. The basic assumption is that eventually the fantasy societies of D&D went through an industrial revolution, where the scientific method was applied to magic, and the outgrowth of that was functionally equivalent to much of the technology of our world without ever getting to the point where magic was just a form of electricity. That is, the principles of magic remain in place, including such things as spell-per-day limits if you’re using a version of D&D that has Vancian magic, but the trappings have all changed to be more in line with the future as imagined back in the 1930’s and ’40’s.