ἆρ᾽ οὖν, εἴπερ ἐναντία ἐστὶν ταῦτα ἀλλήλοις, ἀνάγκη περὶ αὐτῶν ἔχειν ὥσπερ περὶ ὑγιείας ἔχει καὶ νόσου what is the subject of ἔχειν or is it impersonal, or probably τινα is the subj.