Dyad
The first pair. In the Neoplatonic scheme the monad cannot generate numbers at all until a dyad has appeared from it — a claim Eustratius of Nicaea used to argue that the Son and the Spirit must be perfectly matched to each other.
Greek dyas, “a pair”, from dyo, “two”.
The dyad is the first departure from the source — the moment there is more than one thing, and so the moment number becomes possible at all. The surviving sentence of Eustratius of Nicaea puts it as a precondition: the monad "would not step forward into generation of the numbers without previously having the Dyad appeared from it."
Read strictly, that is a claim about a floor and not a ceiling. It says a source cannot get a series going without a pair; it says nothing whatever about where the series stops. A good deal has been credited to Eustratius that his one recovered sentence does not say, and this is the clearest case of it.
Where this comes up
Sources
- Eustratius of Nicaea, Sermon on the Holy Spirit, in Lourié, Scrinium 16 (2020), p. 353