A disorder of the mind
As to the nature of enthusiasm, it is undoubtedly a disorder of the mind, and such a disorder as greatly hinders the exercise of reason. Indeed sometimes it wholly sets reason aside: it not only dims but shuts the eyes of the understanding. It may therefore well be accounted a species of madness — of madness rather than of folly: for a fool is properly one who draws wrong conclusions from right premises, whereas a madman draws right conclusions but from wrong premises. And so does an enthusiast. Suppose his premises true, and his conclusions would necessarily follow. But here lies his mistake: his premises are false. He imagines himself to be what he is not. And therefore, setting out wrong, the farther he goes the more he wanders out of the way.
1771 text
11. *As to the nature of Enthusiasm, it is undoubtedly a disorder of the mind; and such a disorder, as greatly hinders the exercise of reason. Nay sometimes, it wholly sets it aside: it not only dims, but shuts the eyes of the understanding. It may therefore well be accounted a species of madness; of madness rather than of folly: seeing a fool is properly one who draws wrong conclusions from right premises: whereas a madman draws right conclusions, but from wrong premises. And so does an Enthusiast. Suppose his premises true, and his conclusions would necessarily follow. But here lies his mistake, his premises are false. He imagines himself to be what he is not. And therefore setting out wrong, the farther he goes, the more he wanders out of the way.