In SD I, 14 we find
Herbert Spencer has of late so far modified his Agnosticism, as to assert that the nature of the “First Cause,”* which the Occultist more logically derives from the “Causeless Cause,” the “Eternal,” and the “Unknowable,” [...]
where the asterisk refers to the following footnote:
* The “first” presupposes necessarily something which is the “first brought forth, the first in time, space, and rank” — and therefore finite and conditioned. The “first” cannot be the absolute, for it is a manifestation. Therefore, Eastern Occultism calls the Abstract All the “Causeless One Cause,” the “Rootless Root,” and limits the “First Cause” to the Logos, in the sense that Plato gives to this term.
The “First Cause” is the manifested Logos, the Logos proper, “in the sense Plato gives to this term”, which is the Second Logos, as we have shown earlier. (See The Three Logoi)
So the “Abstract All”, the “Causeless One Cause”, the “Rootless Root” is the unmanifested Logos, which we have called the First Logos.