(FORTUNE Magazine) – Let's start with a quick quiz: When is your mother's birthday? How about Thomas Jefferson's? When was Mickey Mouse born? When was the right triangle born?

If you can't answer the first question right away, you know the answer will come to you. If you can't answer the second, you realize it immediately, and you know where to find the response. Because Mickey was created, not born, you know that it's impossible to answer the third question. And you know that the last question makes no sense, because birth and triangles are unrelated.

Notice that you were able to respond to each sentence immediately upon reading it, which says a lot about the difference between human and digital intelligence. You answered more quickly than even the fastest computer. You immediately understood my seemingly simple questions, which presuppose a good deal of common sense and information. You did so without parsing the sentences, sorting through lists of facts, or evaluating alternative responses. In other words, you acted like a person, not a machine.

Keep my little quiz in mind if you're reading the latest book by prolific inventor Ray Kurzweil, The Age of Spiritual Machines, published in January (Viking Press, $25.95). It goes far beyond the old and traditional notion that computers will one day outthink man. In the 21st century, Kurzweil writes, there won't be a clear difference between human beings and robots. Within 20 years, virtual sex will provide sensations that are more intense and pleasurable than conventional sex, and digital prostitutes are likely to be legal. Before the year 2100, machines will pray and worship. Brain scanners routinely will download our histories and personalities into robots. We will become software, and life expectancy will no longer be a viable term in relation to intelligent beings.

Such fanciful speculations, while entertaining fiction, shouldn't be taken too seriously. Still, Kurzweil's accomplishments do command attention and respect. For more than a quarter-century, he's been a pioneer on the computer frontiers--in scanner technology, optical-character and speech-recognition systems, speech synthesis, and digital music synthesis. He holds an MIT degree and several honorary doctorates. A previous work, The Age of Intelligent Machines, was named the outstanding computer science book for 1990 by the Association of American Publishers. And he has founded and sold several successful companies, one while still an undergraduate.

In addition, some of his predictions from a decade ago seem prescient today. He was correct, for example, about the timing of commercially useful speech recognition, and off by only a year in predicting when a computer would defeat the human champion at chess. Despite his fantasies of a post-biological era a century from now, Kurzweil's new book may actually be useful as a forecast of computer industry advances in the next ten years. Let me cite a few examples.

--Most computers will be portable, not desk-bound, hidden in our cellular phones, credit cards, watches, and even our clothing.

--PCs won't have moving parts. No hard drives. No keyboards.

--Instead of typing, most people will talk to their computers.

--In place of PCs in our dens and bedrooms, we'll rely on household servers that will feed digital information to a network of audio and video systems and to booklike displays around the home.

--Vast amounts of information will be wirelessly accessible all the time.

--Music, software, and books will be distributed wirelessly, never assuming a physical form.

--Books, magazines, and newspapers will be read on displays that have the resolution, contrast, and viewing angle of paper.

--Many business transactions will take place between a human and what the author calls a "virtual personality."

The key here is not the direction but the time frame. Kurzweil expects all this within a decade. He may be right.

However, I question his vision and assumptions as he looks further out. As we approach the new millennium and digital technology assumes a more significant economic and social role, it's important to recognize why we haven't built machines that outsmart us, and why trying is a waste of time and resources. Computers prevail at chess, guide missiles more accurately, and provide better stock-trading advice. It will become increasingly appropriate and economical for digital intelligence to assist humans in carefully defined areas like these that don't require general knowledge or the ability to comprehend language.

But the reason computers don't yet have the intelligence of a child isn't that our electronic circuits aren't fast enough. Thought is more than the manipulation of symbols. Contrary to what Kurzweil envisions, the state of every atom inside a living skull will never be determined and programmed into a computer. But if this were possible, the effort would reveal no more of that brain's thoughts than a circuit diagram of a radio reveals about the music playing on it. The real reason we can't yet turn a machine into a mind: We don't know how. We never will.

Richard A. Shaffer is founder of Technologic Partners, an information company focused on emerging technology. Except as noted, Shaffer has no financial interest in the companies mentioned. For an expanded version of Watch this Space, visit www.tpsite.com/tp/fortune/. If you have comments, please send them to shaffer@technologicp.com.