So this guy, Douglas Engelbart. Apparently he invented a bunch of stuff that we use in tandem with the computer and Internet today, from the computer mouse (personally I rarely use a mouse these days, opting instead for touchpads and pen tablets but..) to hypertext.
Also according to this website his ideas were probably inspired in large part by Vannevar Bush's "As We May Think", which makes sense when considering that Bush's ideas, while revolutionary were also stuck in the realm of the "What if?" No surprise then that Engelbart was able to take Bush's theories and apply them to a much cleaner, sleeker input format, even if his original mouse design did look rather ghetto:
Still, a mouse definitely provides a less cumbersome means of interacting with a computer than Bush's desk-hogging assortment of levers and cranks (if you really stop to think about it operating a machine like that would probably take some coaching in itself...although given the various keyboard shortcuts and accessories available for computers today we've probably come full circle in terms of complexity).
An interesting question posed to Engelbart in an interview contrasts the mouse with other forms of navigation, such as touchscreens and "light pens" where he discusses the merits (and specifically precision) of a mouse as compared to the other devices. Certainly this is something that has held true till today, even in the field of PC gaming where professionals tout the advantages in precision by using computer mice.
Conversely however it is also interesting to note how the times and technology have changed since Engelbart's time, as we see devices such as touchscreens and "light pens" (e.g. Wacom's range of casual and professional pen tablets) come into their own in specific or even mainstream uses such as the Nintendo DS, or iPhone/iPad. Of course, this doesn't mean that Engelbart's work and promotion on computer mice was wrong, but it is interesting to note how various factors promote and restrict certain types of interfaces and media, and how these perceptions change over time (as they will inevitably change again after our's). Maybe it'll look something like this:
Nevertheless, all of this belies the fact that Engelbart's overall purpose was less interface and more an attempt to "augment human intellect":
"By 'augmenting human intellect' we mean increasing the capability of a man to approach a complex problem situation, to gain comprehension to suit his particular needs, and to derive solutions to problems. Increased capability in this respect is taken to mean a mixture of the following: more-rapid comprehension, better comprehension, the possibility of gaining a useful degree of comprehension in a situation that previously was too complex, speedier solutions, better solutions, and the possibility of finding solutions to problems that before seemed insoluble. And by "complex situations" we include the professional problems of diplomats, executives, social scientists, life scientists, physical scientists, attorneys, designers—whether the problem situation exists for twenty minutes or twenty years. We do not speak of isolated clever tricks that help in particular situations. We refer to a way of life in an integrated domain where hunches, cut-and-try, intangibles, and the human "feel for a situation" usefully co-exist with powerful concepts, streamlined terminology and notation, sophisticated methods, and high-powered electronic aids." -Internet Pioneers
What this basically comes down to is an attempt to aid people's attempts at knowledge, understanding and the very human desire to think, which may sound familiar because it was exactly what Bush was trying to achieve, in concept, with his mimicry of human thought processes and association via the Memex (remember: Memory Index). Never mind that the actual form and mode of interaction between the Memex and Engelbart's mouse (or even modern devices today) ended up being so very different, the central, repeating concept was to aid what they saw as the natural pursuit of knowledge and understanding (an understandable viewpoint and carry-over from the post-WWII backdrop in which they envisioned these ideas).
The thing is, it's all well and good to want to aid human thought processes, but the question to consider when you actually get down and dirty with it is, "How?" It's here that it becomes interesting to look at the myriad forms of interfaces again and something you get from that is that so many people see "the solution" in so many ways. So maybe there really isn't one answer, but in place of that there may be "one concept" to keep in mind: Naturalness.
What I mean by "naturalness" is the feel of an interface, the feel that whatever way you interact with, the motions, buttons, thought processes involved becomes second nature and more or less an extension of your body and natural range of motion. Note however that a means of interaction that is lacking in naturalness may be re-examined upon the introduction of new technologies (or even improved, cheaper versions of existing ones). One example of this would be the "impractical" use of touchscreens during Engelbart's time (and even a few years ago when Microsoft first unveiled their Surface platform to a lukewarm reception). However, take what is essentially the same technology, shrink the size of the device and you basically have a best-seller in what is essentially the iPad. An interface has gained sufficient "naturalness" to work.