Through a small telescope, a small region of the sky shows us about 2 million stars. On average, it is estimated that there are about 1020 stars. There may be even more out there. If we say that the conditions for life are around one in a thousand stars, we have 1014 stars. If one in a thousand of these have an atmosphere suitable for life, we get 1011. And if one in a thousand of these have produced life, we have 100 million. Take one in a thousand of these as planets that have developed some form of intelligent life, and we get 100,000 planets. We could keep reducing the probability of existence of intelligent life out there, and still reach 200!
One of the basic assumptions of this theory is the assumption that extraterrestrial life is a reality, and this is reiterated in evidence being presented again and again here.
Now, we constantly make the mistake of assuming that life must exist, develop and flourish in conditions exactly like ours. Yet on this planet of ours, we have anaerobic bacteria: these live in environments that possess no oxygen. Germs have also been found in the lethal water that surrounds a nuclear reactor. Experiments that recreated the conditions of Jupiter were also seen to have supported life. Some bacteria live in volcanoes, some eat stones and some produce iron. The point being made here is that life can exist in any condition. An alien civilization out there might consider their conditions to be the ideal one for sustaining life.
