Appetite and desire
Formal Statement
When the conatus is referred to the mind alone, it is called will. When referred to mind and body together, it is called appetite. Desire is appetite with consciousness thereof. We do not strive for things because we judge them good; rather, we judge them good because we strive for them.
In Plain Language
This is one of Spinoza's most striking reversals of common sense. We think we desire chocolate because it is good. Spinoza says: chocolate seems good to us because we desire it. Value is not a property of objects waiting to be discovered — it is a projection of our conatus. What we call "will" is just the mental face of the same striving that, seen from the body's side, is appetite. Add consciousness and you get desire — the first of the three primary affects.
Why This Follows
From ce-10, the mind is conscious of its striving. Spinoza here simply names that striving under different descriptions: will (mental only), appetite (mind-body), desire (appetite plus awareness). The reversal of good-and-desire follows from the fact that conatus is prior to any judgement of value (ce-08: striving is essence, not a response to external evaluation).
Desire is conscious appetite — the first primitive affect, and the origin of all valuation.
If we call things good only because we desire them, is there any objective basis for ethics in Spinoza's system, or is everything ultimately preference?