Elon Musk, who predicted in March 2020 that there would likely be “close to zero new cases” of the coronavirus in the U.S. by that April, has apparently caught the virus once again. The Tesla CEO, who said he’s experiencing almost no symptoms, had previously tested positive last fall. In his tweet on Monday, Musk also waxed philosophical about the changing nature of the virus, invoking the Ship of Theseus, a thought experiment about whether an object retains its original nature after all of its individual components have changed. “How many gene changes before it’s not Covid-19 anymore?” he wrote. Tesla previously came under fire in 2020 for reportedly firing multiple employees who chose not to return to in-person work over concerns about getting sick.