From what little we know of him he was an uncompromising dualist.
If we assume, as we must needs do, that the opinions which Basilides promulgates as the teaching of the "barbari" (Acta Archelai c. 55) were in fact his own, the fragments prove him to have been a decided dualist, and his teaching an interesting further development of oriental (Iranian) dualism.
It follows that philosophy is in a sense both dualist and monist; it is a cosmic dualism inasmuch as it admits the possible existence of matter as a hypothesis, though it denies the possibility of any true knowledge of it, and is hence in regard of the only possible knowledge an idealistic monism.
A dualist he certainly was, but he was not a.
Human birth in a grossly material body is partly due to the pre-temporal fall of souls; here we see in Origen the Greek, the dualist (mind and matter), the ascetic, and to some extent the kinsman of the Gnostics.