Philosophical skepticism comes in various forms. Radical forms of philosophical skepticism deny that "knowledge or rational belief is possible and urge us to suspend judgment on many or all controversial matters." More moderate forms of philosophical skepticism claim only that nothing can be known with certainty, or that we can know little or nothing about nonempirical matters, such as whether God exists, whether human beings have free will, or whether there is an afterlife.

Skepticism has also inspired a number of contemporary social movements. Religious skepticism advocates for doubt concerning basic religious principles, such as immortality, providence, and revelation. Scientific skepticism advocates for testing beliefs for reliability, by subjecting them to systematic investigation using the scientific method, to discover empirical evidence for them.

In ordinary usage, skepticism (US) or scepticism (UK and Can.) can refer to:
  1. an attitude of doubt or a disposition to incredulity either in general or toward a particular subject;
  2. the doctrine that true knowledge or some particular knowledge is uncertain;
  3. the method of suspended judgment, systematic doubt, or criticism that is characteristic of skeptics
In philosophy, skepticism can refer to:
  1. a mode of inquiry that emphasizes critical scrutiny, caution, and intellectual rigor;
  2. a method of obtaining knowledge through systematic doubt and continual testing;
  3. a set of claims about the limitations of human knowledge and the proper response to such limitations.