[1][2][3][4] The concept applies in a variety of disciplines,[5] with both the nature of the evidence or justification and the criteria for sufficiency being area-dependent.
In the area of oral and written communication such as conversation, dialog, rhetoric, etc., a proof is a persuasive perlocutionary speech act, which demonstrates the truth of a proposition.
[19] Suitably incriminating evidence left at the scene of a crime may serve as proof of the identity of the perpetrator.
Descartes famously raised a similarly strict standard with his first principle Cogito, ergo sum (I think, therefore I am).
While Descartes' larger project in Meditations on First Philosophy has knowledge of God and the external world—founded on the certainty of the cogito—as its aim, his legacy in doing so is to have shown that one cannot have such proof, because all perceptions could be false (such as under the evil demon or simulated reality hypotheses).
One nevertheless can still have clear proof of the existence of one's thought, even if belief in the external world lacks the certainty of demonstration beyond that of one's own firsthand experience.