A: Paul Fitts.
Paul Fitts was an Air Force officer who, during the 1950s, developed models of human movements that are now primarily used in human-computer interactions. His model basically predicts the time it takes to move to a target area, such as clicking a button. It could be a physical button, like a "panic button" (ejection seat button) or a virtual submit button on a website.
Bad UI design in aviation can, obviously, be critical and it may have been a key factor in the 2009 Air France crash while enroute from Brazil to Paris.
The basics of Fitts' law is that the closer, bigger, and more obvious the target, the faster and easier it is to interact with it at the appropriate time. The last part is very critical. You don't put the "delete all files" button right next to the "more info" button; nor do you put the ejection seat button next to the "turn on landing light" button. Accidentally interacting with something that you didn't intend to interact with can be just as bad, or worse, than taking too long to find what you're looking for.
Windows windows vis-à-vis Macintosh windows
The basis of Fitts' law is one noticeable theoretical and practical difference between a window on the Mac OS and a window on Windows.
On Windows, the menu bar is at the top of, and attached to, each window. So, the target (menu item) has a specific height (say, 100 pixels). Push your mouse pointer just one pixel past the menu bar and you've missed the target when you click your mouse button.
On a Macintosh OS, the menu bar for the active window is at the very top of the computer screen - there's nothing above it - and, since you can't move your mouse beyond the top of the computer screen, the height of the target is effectively infinite.
Simple and Fast is Good
The simpler and faster it is to do something that you want to do, when you want to do it, the better.
To lock a Windows computer requires two hands in order to press the CTRL-ALT-DEL keys at the same time, followed by the enter key to "Lock this computer." On a Macintosh, to invoke the screen saver which locks the computer only requires moving the mouse pointer to a corner of the screen that you've designed as a "hot corner" (once again, an infinitely large target). As a matter of fact, this technique is a little too easy to invoke, so there's a simple fail safe which allows the users to set how long, after the screen saver hot corner is invoked, before locking the computer (five seconds, one minute, five minutes, etc.).
Putting sequential targets closer to each other is usually better than spreading them far apart - within reason. Also, a touch interface should be treated differently, in certain circumstances, than a traditional desktop since moving a mouse pointer from point A to point B is sequential on a computer screen (the pointer has to pass through every pixel between the two points). Compare this to a real world touch screen where two targets could be tapped in near parallel or with minimal time.
Art vs. Science
While Fitts' law is a quantitative science, there are many other aspects which make the user experience an art. For example, understanding the type of user is important. Sometimes, presenting a casual user with a menu of options to choose from is the right thing. Other times, a computer expert will not want to be limited by a menu of options which is why many of them prefer the command line over a graphical interface for low level OS interactions.
Unlike, say Einstein's theories, which began as theory followed by experimental confirmation; user experience theory usually begins as an observation before being refactored into theorems.