Mr Vod, I agree with you 100%.

There probably are aliens, but we can never know for sure (or even begin to put an estimate on the likelihood of them existing), until we discover alien life on another planet/moon.

For the moment we just have a single data point - life on Earth. And while it's probable that there is other alien life in the Universe, the chances of us even finding it are miniscule.

It's possible that life is abundant in the universe; it's also a possibility that life on Earth was a huge fluke, and we are the only lifeforms in the whole cosmos.

Another factor to consider is the vast timescales involved, and how long an alien civilisation would exist - perhaps there has already been an intelligent alien civilisation that existed billions of years before Earth even formed, and has since become extinct. We may never know...