Daniel said nothing. Isaac’s condition was not improving; he suspected that the gaol-fever was creating a lesion on Isaac’s heart. As a boy Isaac had tried to make perpetual motion machines, seeing in them a model of the heart. But Isaac’s heart, Daniel suspected, was about to give out. Men had not been able to fashion perpetual motion machines because men were mechanics who only knew how to work with inert matter. Hearts pumped longer than any machine could, because the matter of which they were made—or so Alchemists supposed—was suffused with the vegetative spirit.