The Drake Equation:
N is the number of civilizations in our galaxy with which communication might be possible;
R* is the average rate of star formation per year in our galaxy
fp is the fraction of those stars that have planets
ne is the average number of planets that can potentially support life per star that has planets
fℓ is the fraction of the above that actually go on to develop life at some point
fi is the fraction of the above that actually go on to develop intelligent life
fc is the fraction of civilizations that develop a technology that releases detectable signs of their existence into space
L is the length of time such civilizations release detectable signals into space.
The Drake Equation, named after Astrophysicist Dr. Frank Drake, is an argument for the existence of life on other planets with whom we could communicate. The argument is based on probability, essentially arguing that if conscious, communicative life could develop on earth, than the probability is greater than zero that it could have developed elsewhere in our galaxy. While the parameters for the equation are somewhat speculative, we do have plausible value that yield a current estimate of....
In other words, the best guess is that there are 2.31 civilizations in our neighbourhood of the universe capable of communication with us. Of course, if you give different values to the parameters you can make this number go up or down, but it can never be reduced to zero because, in fact, there are people to ask the question!
How cool is that?