No person can know everything. Human language is full of terms like "expert" and "novice" and "car mechanic", which all imply, in one form or another, that people have a different granularity of knowledge on different subjects. Colloquially, the term "know it all" is one of derision - no one can know it all, and those that think so are objects of scorn! It is almost an article of faith that "genius" in one area will mean "fool" in another. People can, however, transfer their knowledge to one another. With the help of a real car mechanic, one might be able to successfully perform one's own oil change, thereby adding to one's knowledge. Not knowing the future is another vital element of any person's incomplete knowledge. Without knowing the future, no one can know what piece of knowledge they will be presented with next.
An oft-repeated example of how knowledge is invalidated by new evidence is the famous swans example. If one only saw white swans, and had no evidence there were any other kind, one could theorise that "All swans are white." Visiting Perth Australia, one would be forced to discard one's theory in the face of evidence which shows that, in fact, some swans are black. On a more complete level, even fundamental knowledge can be cast into doubt by new evidence. In the movie "The Truman Show", the star of the story spends his entire life from birth inside a vast, and completely realistic sound-stage, as part of an ongoing soap opera. Truman is ignorant of this state of affairs, and, suspecting nothing, all his knowledge about his world, those around him, and even god are swept away when a door opens and he steps outside. Similarly, on the television series Star Trek, a device called a "holodeck" can render a virtual reality so realistic that it is indistinguishable from the "real" universe outside. People's senses can be fooled, and for a person such as Truman, or a person in a holodeck with no knowledge of that fact, reality is the reality of the simulation. If and when the "door" to their world is opened, then the reality that they know - in fact all their knowledge, could be invalidated by the "truth" of the soundstage, or the starship Enterprise. Of course, a simulation may exist inside another simulation, making the dramatic irony, or indeed the certainty of any knowledge, a rather more complex issue.
It might seem that there is an escape into certainty for knowledge along one path - completeness of knowledge. If we conceptualise an entity that is omniscient, knowing all things including the future - and the future for that entity does not include a disembodied voice saying "End holodeck program" or indeed a door in the universe opening onto a Hollywood backlot - then perhaps that entity's knowledge can be certain. Fortunately for uncertainty, this omniscient entity by definition is aware of the holodeck idea - and is therefore certain about uncertainty! Perhaps the entity only imagines itself omniscient, or is in fact all-knowing inside a simulated reality, and is simply a creature in another holodrama. Put in another way, can the entity know that it doesn't know? That this is a paradox reflects simply that omniscience is a paradox, and no threat to the uncertainty of knowledge.
As we have seen above, temporal uncertainty, the concept of simulated reality, and the paradox contained within the idea of complete knowledge all show that knowledge can never be certain.
Jostein Gaarder, 1995, Sophie's World, London, Phoenix House
Matthew Allen, Curtin University Audio Tape: The Voice of Reason
David Gerrold, 1972, When H.A.R.L.I.E. Was One, New York: Ballantine
The television programs and movies referenced in the text
Node your homework. In the grand tradition of thinking about thinking, the task for this mini-essay was to make a pursuasive argument for a big, ill-defined topic within an extremely limited word budget, in this case 600 words. Note that the task was specifically not to make a logically complete, or even completely logical argument. Persuasion was the focus, and remains the focus of this slightly edited version, which keeps its original word count of 590 words.
Several people have pointed out that this essay itself asserts certainty of knowledge. Well spotted! This is absolutely intentional. It's a feature, not a bug. It's the central paradox of the whole argument. This essay received the highest mark given in the class, which proves nothing more than that my lecturer and I have the same sense of humour.