It was a custom among our ancestors, practiced even into my own lifetime, to add to the opening words of a letter, “If you are doing well, that’s good; I am doing well myself.” The right thing for us to say is, “If you are doing philosophy, that’s good.” For that is the only way one can really be doing well. Without that, the mind is sick; and the body too, even if it has great strength, is sound only as that of an insane or deranged person might be. So care for the mind’s health first and foremost, and for the other only secondarily: it will not cost you much, if you have resolved to be truly well.
Lucius Annaeus Seneca, Letters on Ethics.
It follows from Seneca's claim that most people are fit, but "only as [. . .] an insane or deranged person might be".