Because we can? We clearly can think intelligently on things besides sex, food, and general survival.
Well, I mean, I can, but we have to beg the question against solipsism in order to have a discussion and I have neither the time nor the will to get into P zombie stuff. 'Sides shit ain't quite germane to the issue at hand.
Now if you meant how can we trust our senses, and other more basic arguments against empiricism, that stuff I'll get into.