Absolute and relative terms
The distinction between absolute and relative terms was introduced by Peter Unger in his 1971 paper A Defense of Skepticism and differentiates between terms that, in their most literal sense, don't admit of degrees (absolute terms) and those that do (relative terms).[1] According to his account, the term "flat", for example, is an absolute term because a surface is either perfectly (or absolutely) flat or isn't flat at all. The terms "bumpy" or "curved", on the other hand, are relative terms because there is no such thing as "absolute bumpiness" or "absolute curvedness" (although in analytic geometry curvedness is quantified). A bumpy surface can always be made bumpier. A truly flat surface, however, can never be made flatter. Colloquially, he acknowledges, we do say things like "surface A is flatter than surface B", but this is just a shorter way of saying "surface A is closer to being flat than surface B". This paraphrasing, however, doesn't work for relative terms. Another important aspect of absolute terms, one that motivated this choice of terminology, is that they can always be modified by the term "absolutely". For example, it is quite natural to say "this surface is absolutely flat", but it would be very strange and barely even meaningful to say "this surface is absolutely bumpy".
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The applicability of absolute terms
Once the distinction is made, it becomes apparent that the application of absolute terms to describe the real-world objects is doubtful. Absolute terms describe properties that are ideal in a Platonic sense, but that are not present in any concrete, real-world object.
For example, while we say of many surfaces of physical things that they are flat, a rather reasonable interpretation of what we presumably observe makes it quite doubtful that these surfaces actually are flat. When we look at a rather smooth block of stone through a powerful microscope, the observed surface appears to be rife with irregularities. And this irregular appearance seems best explained, not by its being taken as an illusory optical phenomenon but, by our taking it to be a finer, more revealing look of a surface which is, in fact, rife with smallish bumps and crevices. Further, we account for bumps and crevices by supposing that the stone is composed of much smaller things, molecules and so on, which are in such a combination that, while a large and sturdy stone is the upshot, no stone with a flat surface is found to obtain.
— Peter Unger, "A Defense of Skepticism"
Certainty and knowledge
The distinction sets up the foundation for the final argument of the paper: that knowledge requires certainty and that, certainty being an absolute term, it follows that it can never be achieved in reality. It is a Platonic ideal that we can get closer and closer to, but never truly reach. In Unger's own words, "every human being knows, at best, hardly anything to be so".
References
- Unger, Peter (April 1971). "A Defense of Skepticism". The Philosophical Review. 80 (2). doi:10.2307/2184030. JSTOR 2184030.