A Skeptic - http://en.wikipedia.org/wiki/Skeptic
In ordinary usage, skepticism refers to:
(a) an attitude of doubt or a disposition to incredulity either in general or toward a particular object;
(b) the doctrine that true knowledge or certainty in a particular area is impossible
In philosophy, skepticism refers more specifically to any one of several propositions. These include propositions about:
(a) an inquiry,
(b) a method of obtaining knowledge through systematic doubt and continual testing,
(c) the arbitrariness, relativity, or subjectivity of moral values,
(d) the limitations of knowledge,
(e) a method of intellectual caution and suspended judgment.