Certain

Certainty is perfect knowledge that has total security from error, or the mental state of being without doubt.

Objectively defined, certainty is total continuity and validity of all foundational inquiry, to the highest degree of precision. Something is certain only if no skepticism can occur. Philosophy (at least, historical Cartesian philosophy) seeks this state.

It is widely held that certainty about the real world is a failed historical enterprise (that is, beyond deductive truths, tautology, etc.). This is in large part due to the power of David Hume's problem of induction. Physicist Carlo Rovelli adds that certainty, in real life, is useless or often damaging (the idea is that "total security from error" is impossible in practice, and a complete "lack of doubt" is undesirable).

Pyrrho is credited as being the first Skeptic philosopher. The main principle of Pyrrho's thought is expressed by the word acatalepsia, which denotes the ability to withhold assent from doctrines regarding the truth of things in their own nature; against every statement its contradiction may be advanced with equal justification. Secondly, it is necessary in view of this fact to preserve an attitude of intellectual suspense, or, as Timon expressed it, no assertion can be known to be better than another.

Al-Ghazali was a professor of philosophy in the 11th century. His book titled The Incoherence of the Philosophers marks a major turn in Islamic epistemology, as Ghazali effectively discovered philosophical skepticism that would not be commonly seen in the West until Averroes, René Descartes, George Berkeley and David Hume. He described the necessity of proving the validity of reason—independently from reason. He attempted this and failed. The doubt that he introduced to his foundation of knowledge could not be reconciled using philosophy. Taking this very seriously, he resigned from his post at the university, and suffered serious psychosomatic illness. It was not until he became a religious sufi that he found a solution to his philosophical problems, which are based on Islamic religion; this encounter with skepticism led Ghazali to embrace a form of theological occasionalism, or the belief that all causal events and interactions are not the product of material conjunctions but rather the immediate and present will of God.

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