Originally Posted by Mal
A single occurrence is not indicative of a trend. An event occurring once does not increase the probability of it reoccurring.
Probability wise, sure. If I flip a coin and it lands heads then that doesn't have any influence on the outcome of the next coin flip. Life existing here doesn't have any effect of any potential life elsewhere. They're all separate events in that respect.
However, if one can prove that life exists somewhere in the universe, then that proves that life existing in the universe is possible. Which means life existing elsewhere in the universe is also possible. Since we already know the universe can support life under certain conditions.
So the fact that life exists in the universe makes it more likely that more life might exist elsewhere in the universe, compared to no life existing anywhere. A single occurrence does present the ability to logically claim the very real possibility of extraterrestrial life.
Now, if we changed the topic to "ghosts" instead of aliens, then the difference is extremely noticeable. Since there isn't any credible proof of ghosts existing anywhere ever. So claiming ghosts might exist would be entirely illogical.
tl;dr-Life existing here does lend credit to the stance that life might exist elsewhere.