The notion that humanity is the only intelligent species in the galaxy or universe is irrational.
First it assumes that other intelligent life forms are the same as humanity or close to it.
Second it assumes that other intelligent life forms want to leave their home world and travel to meet, well us. Or since humans love killing each other another sentient species would arrive with the intention of starting a war of conquest. You know travel umteen hundred light years passing millions of habitable world just to do what humans would do to them if we could.
Third it assumes that an intelligent life form would by nature seek the knowledge of space travel.
Fourth it assumes that a sign of intelligence is the wanton destruction of a species home world brought on by severe overpopulation, since that’s what we’re doing.
It does seem that some humans are actually arrogant enough to feel certain we’re it when ‘it’ is an intelligent life form. Humans as an example of an intelligent life form is one that is destroying its home world. By the end of this century it is quite possible that life on this planet will not be possible due to the extreme lack of resources such as clean drinking water and food. The oceans (Baltic Sea for starters) are showing signs of dying. No oceans, no life.
How’s that for signs of intelligence? Elected officials make every effort to ignore scientific truth so in the end they add a large quantity to their personal fortune. Life be damned. The future can go to hell.
Because we are ignorant enough to disbelieve the factual evidence of our planet wide destruction, surely can we be certain every other intelligent life form that might exist on another planet was, is, or will be the same.
Two examples show that as incorrect. Elephants and dolphins.
If another planet’s sentient species is aquatic, would that care about space flight? If it was avian, would it care about intergalactic travel? Traveling to find a species like us? Seriously?
Living as we do is not necessarily the only sign of intelligence or any kind of indication of intelligence. In fact it may be a sign of a species longing for the end of it all. We’ve included that scenario into most religions as a ‘just in case’.
Believing we are the only sentient species in the galaxy, the universe is ignorance wrapped in a candy coating of arrogance. Period.