Sure, you can talk to people and they can assure you that they do think and are a fully sentient creature just as you are. But still, you could programme a robot to say that as well, but it wouldn't make that true. I'm not too sure I've explained my point very well, but I'm basically saying that you have no way of knowing that there is any other intelligent life in the universe other than yourself. Things can appear to have life, when really they don't at all, and they could just be the case for everybody but you.
There's no way for human beings to share their thoughts other than to talk, you can never experience another’s consciousness. So perhaps, really, yours is the only one. You think you have close friends, but secretly, they're just mindless things which appear to be the same as you. You'll always be alone, even when you're not.
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Not to be pessimistic of course! :) I don't believe this is true. But, nonetheless, it is a philosophical possibility!