Perfection

Perfection would be nice. In fact, it would be perfect. But everybody knows that nobody can be perfect although anybody could see that somebody must be pretty close, because of natural variation.

"Nobody's perfect."
"We're only human."
These are ancient proverbs passed down from the past.
They also happen to be degrading and would be severely detrimental to society if everyone knew it.
These kinds of phrases emphasize the imperfection of humans, which is perfectly legitimate. However, the context in which they are used are, at least 3 times out of 5, as an excuse for a less-than-satisfactory result of something, such as a grade, a project, a software program, or a car. None of these things are perfect, and sayings like those above are sometimes used to waive the consequences.
The key point to realize is that although the state of perfection is exclusively theoretical, we still ought to strive to acheive it. For example, only a few people in the class get an A+. That doesn't mean that the rest of the class does not want or should not try to get an A+; it probably means that they will have to work harder to achieve that high grade. Nobody is perfect, but anybody can try.