It is said that the optimist may view a cup of fluid as half-full, and the pessimist may view it as half-empty. Both may be correct, but there is more to this.
As a rational optimist, I believe in reason and the attempt to get at what is true, but I believe in choosing the more optimistic interpretation when there is room for choice.
First, there are data and definitional uncertainties: there is some uncertainty as to the percentage of a “full” cup that is occupied by the fluid.
What is “full”? Do we measure from the top (the rim) of the cup to the bottom or from the usual height when we consider the cup effectively full? An eight-ounce cup has eight ounces of fluid at a level somewhat below the rim.
Would 51% of capacity warrant “half-full” and 49% warrant “half-empty”? How about “nearly full” and “nearly empty”? And how well do we know that 51% is the right measurement? Perhaps the truth is 55% or 45%, for example.
In some circumstances, not knowing the truth can be dangerous (think of dispensing powerful medicines), so a rational optimist wants to know what is true, when it can be known with reasonable effort.
In other circumstances, the impact on the perceiver may be important, and “half-full” may be more or less encouraging than “half-empty,” and encouragement (or discouragement) might be better.
I choose to be optimistic where the data allow it and confidence is beneficial. Even so, over-confidence is by definition mistaken and can be dangerous.