When Vieussens the Younger explained that the 'oval centre' in the brain was 'the seat of the functions of the spirit', because 'arterial blood there became sufficiently refined to become the animal spirit', and that consequently 'the health of the spirit in its material form depends on the regularity, the equality and the free passage of the spirits through these tiny canals', Fontenelle refused to recognise all that was immediately perceptible and decisive in a criterion so simple, which would allow an immediate division into the mad and the non mad.