This double negative construction follows verbs expressing fear or apprehension, introducing a feared negative outcome. Homer uses it: 'I fear lest someone not undertake this work for you' (meaning 'I fear no one will'). It appears most frequently with the subjunctive, though the indicative is also attested. Sometimes it introduces hesitation without an explicit introductory verb. The construction captures the psychological complexity of fearing a negative: 'afraid that... not.'