According to the website uncontactedtribes.org, there are about 100 tribes left in the world that have not been contacted by mainstream society.
Imagine being someone in one of these tribes, and a visitor tries to explain cars to you, when your concept of machines is limited to simple things like bows and arrows. "A car is a bit like a house, except that it is much harder, and it moves kind of by itself, but takes in fuel, which is like eating, but it is all liquid and is made from things in the ground. You get inside it and it can bring you places." You can imagine that, as this description continued, it would begin to sound absurd.
When things sound absurd we tend to not believe them. As you might imagine, trying to explain the modern world to someone from an uncontacted tribe would probably result in denial and disbelief. Imagine being an adult in such a society. You feel you've expeienced the world. You've seen a lot of forest, met people from other tribes. How could such a society exist and you not know about it? If they truly had these amazing driving things, where are they? Why haven't we or any of our grandparents ever heard of them? They really couldn't be bothered to come over and visit?
Indeed, you could argue that you would be rational to disbelieve in the advanced society being described to you. It just doesn't make sense, given what you know of the world.
Yet, in spite of there being people who, possibly rationally, disbelieve in the modern world, the modern world exists.
Now think about us. Could there be another civilization that is significantly more advanced than ours that we don't know about? It's hard to imagine. We have explored most of Earth. If they had aircraft we would have encountered them by now, right? They can't be hiding, can they?
Perhaps there are extraterrestial societies with technology far beyond ours, who just have not visited us yet, for one reason or another. What would their society be like? Science fiction authors have tried to describe technologically advanced societies, but it has to be done in a way that we, as readers, can understand. But if the tribespeople cannot understand cars, given what they know, perhaps there is advanced technology out there that would be equally unimaginable to us. That is, if someone were to describe it to us, we would not understand it, and probably disbelieve the report.
The engine of imagination uses our memories as fuel. What we know limits our imagination, but also makes imagination possible in the first place. We are only able to imagine recombinations of ideas and concepts we already have—sometimes these are very sophisticated and abstract concepts, but it is hard to come up with revolutionary, novel things for this reason. That kind of novelty takes generations of people building on ideas in the cultures they are born into.
One of the arguments against the existence of intelligent extraterrestrial life is, basically "well, where are they?" The same argument could be made by the undiscovered tribesperson. Well, let's look at the reasons we still have undiscovered tribes on Earth. Our diseases would kill them, so we have some good reason not to contact them. But for the most part, I suspect the real reason we have not contacted them is because it has not been in anyone's interest to do so. In other words, we just haven't bothered.
Could it be that there are intelligent extraterrestrial societies who just haven't bothered to contact us? The idea is mildly offensive. The idea that Earth, humankind, with all of our culture, our ideas, the great views in Bryce Canyon, are just not interesting enough to come and look at? As strange as that sounds, that is the main reason we have tribes yet to contact, with all of their language, culture, and everything else they have, all of which is probably very interesting.
So are there smart aliens? We don't know.
However, we know enough about the human mind to know that using our imagination to guess what technologically advanced extraterrestials are like is going to be severely limited.
Pictured: An uncontacted tribe in Brazil. From Wikimedia Commons.