“Look, if you want to drift into hell on Earth, stay 24 months in a consulting firm and you are tainted meat for the rest of your life,” O’Leary tells Fortune. “No one’s going to hire you to make a decision because you never have made one.”

“Why would anybody burn all those hours while someone else makes money, and you do nothing of consequence? I respect all the consulting firms that are out there, but I’m going to do my best to keep people from going into that.”

O’Leary is an Executive Fellow at the prestigious Ivy League college, teaching an MBA Elective Curriculum course The Founder Mindset. The school has pumped out some of the most successful entrepreneurs, including Michael Bloomberg, Sheryl Sandberg, Jamie Dimon, and Bill Ackman—but the institution’s entrepreneurial spirit hasn’t rubbed off on all of its students. O’Leary says that when he asks his Harvard cohort who wants to go into consulting, about two thirds raise their hands. But the $4.2 billion SoftKey Software Products founder has made it his mission to recruit them into entrepreneurial life—even if his lessons require some harsh truths. 

“What I try and do is disrupt a few of them in every class that I go into at the beginning of the program saying, ‘If I can get four of you to abandon your drift into mediocrity, then I’ve done a great job here.”

Business consultants will ‘never be free’

O’Leary understands the draw to the consulting world; he notes these jobs can offer $250,000 to $350,000 salaries right off the bat, despite consultants being worked “like an animal” for the first three years. But sky-high wages and cushy offices might not be worth the price they have to pay: never producing anything of their own, always working for the big man and putting off potential employers, like O’Leary.

“If you’re there for more than two than 24 months, you get the virus. You’re tainted—your resume says you were someone of no consequence,” the 71-year-old tells Fortune.

“So I always take those resumes of consultants that want to get into the real world, and throw them in the garbage,” O’Leary continues. “They haven’t done anything, they just wrote reports. Didn’t matter.”

Being worked to the bone for six-figures is still enticing for some professionals, as many log in 100-hour workweeks for much less. But beyond having an underwhelming resume, O’Leary says consultants will never have freedom working under a boss. 

“You can go to the soccer games, go to picnics. You can do whatever, and it’s a great life. You can provide for a family,” he adds. “But you’ll never be free. You’ll never be financially free.”

Entrepreneurship may mean no vacations, sharing an apartment with five roommates, and grinding for years—but once you make it, you can call your own shots.