Skepticism is generally a questioning attitude or doubt towards one or more items of putative knowledge or belief or dogma. It is often directed at domains, such as the supernatural, morality (moral skepticism), theism (skepticism about the existence of God), or knowledge (skepticism about the possibility of knowledge, or of certainty) Formally, skepticism as a topic occurs in the context of philosophy, particularly epistemology, although it can be applied to any topic such as politics, religion, and pseudoscience.
Philosophical skepticism comes in various forms. Radical forms of 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 skepticism claim only that nothing can be known with certainty, or that we can know little or nothing about the big questions in life, such as whether God exists or whether there is an afterlife. Religious skepticism is "doubt concerning basic religious principles (such as immortality, providence, and revelation). Scientific skepticism concerns testing beliefs for reliability, by subjecting them to systematic investigation using the scientific method, to discover empirical evidence for them.
As a philosophical school or movement, skepticism arose both in ancient Greece and India. In India the Ajñana school of philosophy espoused skepticism. It was a major early rival of Buddhism and Jainism, and a major influence on Buddhism. Two of the foremost disciples of the Buddha, Sariputta and Moggallana, were initially the students of the Ajñana philosopher Sanjaya Belatthiputta, and a strong element of skepticism is found in Early Buddhism, most particularly in the Atthakavagga sutra. Since skepticism is a philosophical attitude and a style of philosophising rather than a position, the Ajñanins may have influenced other skeptical thinkers of India such as Nagarjuna, Jayarasi Bhatta, and Shriharsha.
In Greece philosophers as early as Xenophanes (c. 570 – c. 475 BC) expressed skeptical views, as did Democritus and a number of Sophists. Gorgias, for example, reputedly argued that nothing exists, that even if there were something we could not know it, and that even if we could know it we could not communicate it. The Heraclitean philosopher Cratylus refused to discuss anything and would merely wriggle his finger, claiming that communication is impossible since meanings are constantly changing. Socrates also had skeptical tendencies, claiming that he knew nothing, or at least nothing worthwhile.