Actually, we know almost all basic chemistry, and the range of (stable) molecules that silicon can form is orders of magnitude less than for carbon.
Well, yeah, but I didn't want to offend the pedants even further. Unless the laws of physics (and therefore basic chemistry) are very different elsewhere in the galaxy, it's not unreasonable to think that carbon-based, liquid-water-dependent lifeforms are the most probable. In fact, I'd be willing to bet a tidy sum of money that the overwhelming majority of unique forms of life are not terribly dissimilar from ours as far as the underlying chemistry is concerned. They might be fantastically alien in all sorts of other strange ways, but they'll still be based on simple organic polymers. But this is still irrelevant to the discussion at hand, because even if there were different forms of life, we have no idea how we might detect them at astronomical distances.