Full Idea
When striving [conatus] is related to the Mind it is called Will, but when related to the Mind and Body it is called Appetite. This Appetite is the essence of man, from whose nature there necessarily follow those things that promote this preservation.
Gist of Idea
The conatus (striving) of mind and body together is appetite, which is the essence of man
Source
Baruch de Spinoza (The Ethics [1675], III Pr 09S), quoted by Stephan Schmid - Faculties in Early Modern Philosophy 3
Book Reference
'The Faculties: a history', ed/tr. Perler,Dominic [OUP 2015], p.169
A Reaction
Spinoza seems to see 'conatus' as a fairly unified thing, where Nietzsche sees the will to power as a combination of many competing 'drives'. I think Nietzsche is closer to the truth.