Isostheny

In ancient Greek skepticism, particularly in Pyrrhonism, isostheneia (often rendered in English as "isostheny") refers to the notion of equal strength or equipollence between opposing arguments or perceptions. Pyrrhonist skeptics, such as Sextus Empiricus, observed that for virtually any philosophical claim or sensory impression, one could produce a counter-argument or conflicting impression of equal persuasiveness. Because each side of a question appears to have arguments of comparable force, the skeptic is led to epoché (the suspension of judgment).

The guiding idea of isostheneia is that no single position enjoys a decisive advantage over its opposite once all relevant arguments and evidence are considered. As a result, the mind, confronted with equally convincing but contradictory perspectives, refrains from asserting any particular standpoint as definitively true or false. This state of equipoise, according to Pyrrhonists, can foster a form of tranquility: by withholding judgment, one avoids the dogmatic anxiety that comes from trying to establish certainty where none seems attainable.