Εἴδη

The Pythagoreans used to say that the monad and dyad and triad are principles of the forms, since those who arrange from the monad up to the tetrad produce the decad; and they also regarded it perfect and named it 'receptacle'.¹ So they said that these tetradic principles existed in everything, both in general and in particular, and in the domain of the intelligible as well as in the physical and sensible things. Now the intelligible things are a monad in all respects (for it is understood not only that their substance is lacking parts but also that their activity is at rest and in absence of movement),² and knowledge or objects of knowledge are also a dyad. For [this is] what according to its definition goes from something toward something, for knowledge is a transformation from what is definite toward what is definite.³ For there is no knowledge that is indefinite; this is, in fact, how knowledge gets its name: because it leads us to [a state of] rest.⁴ That which is the object of opinion and that which is physical are triads, and I call the universals 'physical things', I mean the ones that are in the domain of physical things, but not the particulars. For opinion [concerns] the universal physical things. Therefore, opinion is a triad since it starts from something, and it does not go to something in a definite way but in this way or that.