  Intellect is the principle of essence or whatness or intelligibility as the One is the principle of being.  Intellect is an eternal instrument of the One’s causality (see V 4. 1, 1–4; VI 7. 42, 21–23). The dependence of anything ‘below’ Intellect is owing to the One’s ultimate causality along with Intellect, which explains, via the Forms, why that being is the kind of thing it is.  Intellect needs the One as cause of its being in order for Intellect to be a paradigmatic cause and the One needs Intellect in order for there to be anything with an intelligible structure.  Intellect could not suffice as a first principle of all because the complexity of thinking (thinker and object of thought and multiplicity of objects of thought) requires as an explanation something that is absolutely simple.  In addition, the One may even be said to need Intellect to produce Intellect.  This is so because Plotinus distinguishes two logical ‘phases’ of Intellect’s production from the One (see V 1. 7).  The first phase indicates the fundamental activity of intellection or thinking; the second, the actualization of thinking which constitutes the being of the Forms.  This thinking is the way Intellect ‘returns’ to the One.