My favorite icism? I think it might be...

Skepticism

Skepticism comes in two primary forms:

Philosophical skepticism is a philosophical position in which people choose critically to examine whether the knowledge and perceptions that they have are actually true, and whether or not one can ever be said to have absolutely true knowledge.

Scientific skepticism is a scientific, or practical, position in which one questions the veracity of extraordinary claims, and seeks to prove or disprove them using the scientific method.

Additionally, the word "skeptic" is commonly used in today's vernacular to describe someone who habitually doubts commonly accepted ideas.

From Wikipedia