illusions - senses might be decieved E.g stick appears bent in water
dreams - might still be dreams E.g vivid dreams
Evil Demon - all our perceptions might be being decieved by a supreme power (Evil demon)
problem of induction - cause and effect in the world may not happen the same way in the future as it did in the past
a priori concepts - somethings can only be known or are better known a prioir (rationally)
contingency - empricism isn't certain as it could be otherwise. opposites don't imply a logical contradiction, unlike rational statements which are necessary mean they are certain, opposites imply logical contradictions.