In a newsletter released today, the Quantum (NASDAQ:) Economics founder said Bitcoin’s recent rally past $23,000 — a gain of roughly 29% in a week — didn’t necessarily mean the market was “overheating” and heading for a crash like the one in December 2017.

“In our opinion, things are just getting started,” said Greenspan. “We probably shouldn’t be using any old yardsticks when it comes to determining what kinds of percentage gains are possible.”