How do i know that i dont have down syndrome/some similar mentally debilitating condition and i just dont know it?
I figure if i did have one i wouldnt realize it, like that kid in your class who doesnt realize hes different from everyone else. Even though i think my behavior is similar to others' I could just be mistaken as a result of the condition.
Often a really good think that happens to me will make me think its impossible to actually be happening and thus I'm just defective and imagined it or something. Or everyone is just being extra nice and playing a fun game to make Marcus the mongoloid feel better about himself.
PS: despite some similarities i dont think cogito ergo sum works here because the thinking is what is in question