David Ferrucci, lead researcher for IBM's Watson project, on designing a machine that can understand human language.

Natural-language understanding is hard for a computer. Our beloved language looks like just a bunch of symbols to a computer, ultimately a long series of 0s and 1s. The meaning we associate with words, phrases, sentences, or entire compositions is grounded in human experience. We use language to capture and communicate human knowledge that makes sense only because of how the symbols in our language are grounded in a common experience.

For example, what does the word bat mean to you? I imagine lots of ideas are flying through your head, and you are wondering about the context in which these words are used and linking them to your personal experiences. As I give you more context, you may start to zero in on its intended meaning.

How about this: "The bat flew through the window."

Now at least you know that I am using bat as a noun and did not intend it to be a verb, as in "to bat around some ideas." But am I referring to a flying animal of the suborder Microchiroptera, or to a heavy stick used to hit a ball? More context is needed. If I added "and then Johnny came running home," then what would you think?

Our example with a bat is an easy one. Consider the word interesting. What does that conjure up for you? For me it may be very different, and yet we can both agree on the emotional or psychological response interestingness creates for people regardless of what they find interesting. This is uniquely human.

Human language is very rich. It can express the same idea in almost an infinite number of ways. Consider that these statements for purposes of diagnosis may be equivalent: 

The patient could not get anything down without pain.

The patient demonstrated persistent difficulty swallowing.

And that those statements contradict these other two:

The patient eats without discomfort.

The patient shows no signs of dysphagia.

Computers are not humans. They cannot map words and phrases to human experience and quickly look for the right contextual clues to understand precisely what is intended. All they have are the symbols, not their human meaning.