G.D.P. is not a measure of how much value is produced for consumers. Everybody should recognize that G.D.P. is not a welfare metric.

It's most useful to think about not jobs but tasks. And within any given job, there are lots of different tasks. If you're a radiologist maybe reading the images machines can be able to do that better, maybe making the broader diagnosis and communicating it to the patients.

We're rapidly entering a world where everything can be monitored and measured. But the big problem is going to be the ability of humans to use, analyze and make sense of the data.

The kind of job where you come in and work 9 to 5, and where someone tells you what to do all day is becoming scarcer and scarcer.

Now comes the second machine age. Computers and other digital advances are doing for mental power - the ability to use our brains to understand and shape our environments-what the steam engine and its descendants did for muscle power.

Electricity is an example of a general purpose technology, like the steam engine before it. General purpose technologies drive most economic growth, because they unleash cascades of complementary innovations, like lightbulbs and, yes, factory redesign.

Machines already are much smarter than us at so many things. I mean, try to multiply two 10-digit numbers with each other or, you know, sift through a thousand documents. So there's lots of things that machines are better at including in mental task than us. There's many more that they're not as good at, but the direction is pretty obvious and the progress is clear.