It is very egotistical of us to believe that we are the only sentient life form in the whole universe, or universes. And that any life must be carbon and water based like we are.
It might exist in ways we don't comprehend- yet.
Yup
Considering the fact we base the requirements for life on a single planet's example means we don't really know the requirements for life in the Universe.
We understand the requirement for life on this planet as far as we can study.
Since we have rather recently found life at black smokers, where no life should be, is evidence we do not even understand the requirements for life on this planet.
We can only try to understand life as we find it, observe it and test it.
Our observations is limited to our domain.
Out testing is limited to our ability.
What we don't know about the Universe greatly exceeds what we do know.
The same applies to intelligence.
We base the definition of intelligence on what we understand about it.
Human civilization is a mere 5-10,000 years old.
There may be intelligent civilizations in the Universe which exceed 100 million or more years. We don't know because we haven't encountered any, yet.
We may not even understand the definition of intelligence on our own planet.
We base the definition of intelligence on our own example.
We assume we are the most intelligent species on this planet but we don't KNOW because we don't understand every species on this planet.
Hell, plants might be more intelligent than humans.
Its all relevant to our own observational limits. Our own values. Our own time frame.