The theory of sign and proof is an essential component of Stoic epistemology. This book examines the fragmentary evidence from Sextus Empiricus and sheds light on the two aspects that characterise signs and proofs: the logical relation that holds between a sign and that which it signifies and an additional epistemic relation that is called revelation. All signs feature in conditionals that are true in virtue of the strong modal account of conditionals that the Stoics developed. This modal account is one aspect that makes it possible to gain certain knowledge of that which is signified. But signs and proofs must also reveal that which they signify to produce knowledge. This requirement is fulfilled when an inference proceeds from premises that are justified through knowledge of the causal relation in virtue of which the premises are true.

Fabian Ruge studied philosophy and Latin in Halle and Berlin and completed his doctorate in ancient philosophy in 2021 in Munich. He holds a position as research associate at Ruhr University Bochum.