NEW YORK (FORTUNE) - The landscape of business altered last week, when Bill Gates said that he will leave full-time work at Microsoft in 2008. More than any other person, he defined the last twenty years in business.

For most of that time, if FORTUNE wanted to maximize its sales, we had merely to put Gates on our cover.

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Ballmer ain't goin' nowhere! As Bill Gates announces his Microsoft retirement, some on Wall Street are calling for the CEO's resignation. But he will remain firmly in charge, and that's a good thing. (Read the column) More Fast Forward Ask.com: Google's up-and-coming rival All Web sites are alike The Net's not-so-secret economy of crime Fast Forward archive

But I think Gates, as always a uniquely acute analyst of business and societal trends, recognizes that his era as a corporate leader and a business strategist has passed. There is a new opportunity for which he is perfectly suited - leading business toward more social consciousness.

More than any other individual in business - by far - he transformed the way all of us live our lives. By adopting and resolutely sticking with Microsoft's founding mantra - "A computer on every desk and in every home" - he and his company achieved what he intended. They made the individual more powerful.

"Everything we've done is of the theme, which is software empowerment," he told me in March.

In succesfully focusing Microsoft (Charts) - and the technology industry - on the opportunity to empower the individual with the PC, Gates also laid the groundwork for the Internet era. If the PC hadn't become pervasive, there would have been no way or reason to use the Internet.

But now Microsoft has to completely transform itself to become a true Internet company - transforming its very business model so that its revenues come more and more from advertising and subscriptions associated with Internet services rather than from selling a discrete piece of PC software. This is not the world Gates understands. So far, the Internet services era has been dominated by Google (Charts), not by Microsoft.

I strongly foreshadowed the shift in Gates' responsibilities in my story Microsoft's New Brain in FORTUNE's May 1, 2006 issue. The article explains how Gates is handing the technological reins of Microsoft over to Ray Ozzie. It opens with an account of a key strategy meeting of all the company's top leaders in June 2005.

At that meeting, the executives decided to move aggressively to the advertising and services model. But Gates was absent. The meeting was chaired by Ozzie, an outsider who had only recently joined Microsoft (but who, of course, has Gates' complete confidence).

The most portentous passage in the story was this one: "[Gates] doesn't say so, but some who know him well say he...appears to be more engaged with the global-health work of the Bill and Melinda Gates Foundation than with Microsoft. Does Ozzie's growing role at Microsoft mean Gates' is declining? Asked that question, Gates hesitates. 'The big-picture strategy here at Microsoft isn't just a single-person job,' he begins." That hesitation, which was surprisingly long for a guy never at a loss for words, spoke volumes.

I think Gates realizes he has neither the mindset for the new Internet-services era nor sufficient passion for the enormous effort needed to help Microsoft successfully transition to it. Who can blame him? He already changed the world once.

But he is not going quietly into the night. This guy does nothing halfway. He wants to make an impact in global healthcare comparable to the one he made in business.

Gates can not be both the world's greatest philanthropist and the man who saved Microsoft yet again. Those jobs are too big. His foundation is already the world's biggest by far.

It is already leading the charge in tackling the worst tropical diseases afflicting the developing-world poor. Yet with Bill's fulltime attention devoted to it - watch out. Nobody I've ever seen or known has comparable focus or intellectual discipline.

That Gates is leaving daily responsibilities at Microsoft to devote most of his time to health care and social betterment makes me wonder if he will now define the next phase of modern business. Nothing would be better than if his departure helped inspire the global business community to finally, resolutely, realize that if it doesn't make this a more liveable planet, there may not be much future for capitalism.