johnmc said:
just purely because of the size of the universe and the number of stars?
I tend to come to a balance between the the mediocrity principle and the Fermi Paradox. There is no reason to assume that the earth is special. It's composition and orbit are nothing out of the ordinary, nor is our star, its composition, output, and its position in the galaxy and the universe. By the extension of this principle, when you consider the sheer number of stars and therefore planets, you can come to the conclusion that there must be many earth-like planets out there as an absolute figure.
However, it also appears, to me at least, that our galaxy is not teaming with intelligent life (intelligence for the purpose of the Fermi Paradox being defined as the ability to communicate at distances of a galaxy or more through any means). Our planet has had thousands upon thousands of species in its billions of years and only one has reached that level.
So, with that in mind. I believe life will be common throughout our galaxy and the universe. Indeed, it's probably in our own solar system e.g. Titan, but intelligent life will be exceptionally rare, but not so rare so that we'd be alone.