It's a cute quote, but not entirely accurate. The human body is intuitive. How do you use your eyes? How do you digest food? How do you use your brain to learn?

Intuition: The act or faculty of knowing or sensing without the use of rational processes; immediate cognition.

We use alot of labels to describe how easy a system is to understand and learn. Obvious or invisible intgerfaces are close cousins to intuitive. I would describe obvious as an interface that seemingly has no other possible interpretation of how to use it. Like a door with a flat plate should be pushed (not pulled). It's obvious because there really is no other choice. Invisible is an automatic light that turns on when you enter a room. Nothing to think about at all. Just being there is enough.

To me intuitive (in the common usage) is an interface that gives you enough hints to make it easy to learn via muddling. In other words, it doesn't make you do anything seemingly unnatural. All the of the gestures are ones you might guess if you just tried to use it. An iPhone is very much like that. It mimics the 3D world.

I try to make my systems intuitive by thinking of all the ways someone might guess how to use it and making all of those gestures work. When people guess and they are right, they call it intuitive.

Lastly, regarding the nipple...it has many more uses than milk. I wonder which use he had in mind. ;)

“Intuitive” (technically, it should be “intuitable”) means the user can use the UI without having to consciously stop and figure the UI out. Learned habituated responses are performed without conscious thought, so intuitive includes more than instincts.

Intuitive is desirable because the less the user has to think about the UI, the more they can focus on the task and complete it quickly without error. Like “user-friendly” and “simple,” it’s a term laypeople understand well, but among us UX professionals, it’s too broad to be useful in most of our own communications. The following more specific UI features constitute intuitive:

• Affordances. UI elements have a natural shape and position that communicates the expected interaction. Nipples fall into this category.

• Clear Labels. UI elements have short clear labels that communicate in the user’s own language.

• Organization. Similar and related items are proximal to each other in the UI.

• Compatibility. Positions and directions of motions are consistent with UI responses. For example, to move my pointer from my left to right monitor, I slew off the right side of my left monitor.

• Feedback. The effects of user actions are immediately apparent where the user is focusing attention.

• Appropriate metaphors. The UI makes explicit associations to physical and cultural analogues and the UI behavior is consistent with that analogue.

• External consistency. Given one thing in your UI and another thing outside your UI that the user knows well, if the two things appear the same then they have the same meaning; if they have the same meaning, then they should appear the same.

The last one might be arguably called the “grandfather” feature of all that is intuitive, where the other features are corollaries of it. This implies that designing to be intuitive primarily means leveraging user learning and experience (although it also includes leveraging human instincts). Thus, how intuitive your UI is depends on your users, and the experience and knowledge they bring to it.

"Intuitive" means

• A user does not have to use a manual to figure out how to complete a task.
• A user does not need to learn through trial-and-error.
• The interactions are not the focal point. It is not hide-and-seek. Interactions are subtle tools to reach the goal.

From these vague points it is clear that interfaces are depended on the target group. If the target group of an interface is the whole world, then it should be as close to the mental model of the real world interaction as it can be. Since this by definition is a limiting factor, be sure that there is no universally recognised and appreciated User Interfaces - only interfaces appreciated by large groups of people.

Even the nipple is not recognised by people that have not been breast fed. Also, the whole system behind it unstable, has different context depending on the age of the owner and the user and it may behave differently under certain circumstances. Bad interaction example :)

Ah, I think the cash machine example is wrong - I'm old enough to remember when our banks introduced them. They had a sales rep out the front of the bank giving customers lessons, and uptake with older people - people who you would think would be able to work these things out - didn't like them and resisted early efforts to learn.

Common place artefacts are always said to be intuitive because they have always been a part of your life and one tends to forget the initial learning of a process.

I've always thought that you learn throughout life including that, when you are a baby, that nice big pink thing gives you yummy food!

This quote is cute, often cited, and actually simply wrong. Even the nipple (as a feeding ‘device’) is learned – just ask some midwives and dry-nurses how many young mothers struggle with teaching(sic!) their newborns how to drink.

Taking this into account, one should rather reconsider the concept of ‘intuitive’. See e.g. Glen's and Michael's answers.

An example I was given ages ago was a cash machine ATM. Nobody has ever been given instructions how to operate one, they just go to it and use it. That may not be quite as applicable nowadays, now that there is more to ATMs than just withdrawing cash (now you can top up phones etc.) but I think it still makes its point.

You could also discribe a pelican crossing terminal as intuitive. Just one button, and we know how to use that instinctively too.