Full Idea
Every substance is indivisible and consequently every corporeal substance must have a soul or at least an entelechy which is analogous to the soul, since otherwise bodies would be no more than phenomena.
Clarification
An 'entelechy' is what makes something actual, or perfect, not merely potential
Gist of Idea
Bodies need a soul (or something like it) to avoid being mere phenomena
Source
Gottfried Leibniz (Letters to Antoine Arnauld [1686], G II 121), quoted by Daniel Garber - Leibniz:Body,Substance,Monad 2
Book Reference
Garber,Daniel: 'Leibniz: Body, Substance, Monad' [OUP 2009], p.88
A Reaction
There is a large gap between having 'a soul' and having something 'analogous to a soul'. I take the analogy to be merely as originators of action. Leibniz wants to add appetite and sensation to the Aristotelian forms (but knows this is dubious!).