be at fault

From LSJ

Ποιητὴς, ὁπόταν ἐν τῷ τρίποδι τῆς Μούσης καθίζηται, τότε οὐκ ἔμφρων ἐστίν → Whenever a poet is seated on the Muses' tripod, he is not in his senses

Plato, Laws, 719c