Spinoza's Ethics: III.P28 - Following Beliefs For Power

Spinoza argues here that we act on what we imagine to increase our power, our joy and that we act against what we imagine to decrease our power, our joy. This conatus is our nature and follows accordingly. When people act, they act logically on their beliefs in accordance with the power of their beliefs. 

Id omne quod ad lætitiam conducere imaginamur, conamur promovere ut fiat; quod vero eidem repugnare sive ad tristitiam conducere imaginamur, amovere vel destruere conamur.

Everything which we imagine to lead to joy, we try to promote so that it happens; that which truly we imagine to repulse (joy) or lead to sadness, we try to remove or destroy.

DEMONSTRATIO: Quod ad lætitiam conducere imaginamur, quantum possumus imaginari conamur (per propositionem 12 hujus) hoc est (per propositionem 17 partis II) id quantum possumus conabimur ut præsens sive ut actu existens contemplari. Sed mentis conatus seu potentia in cogitando æqualis et simul natura est cum corporis conatu seu potentia in agendo (ut clare sequitur ex corollario propositionis 7 et corollario propositionis 11 partis II) : ergo ut id existat absolute conamur sive (quod per scholium propositionis 9 hujus idem est) appetimus et intendimus; quod erat primum. Deinde si id quod tristitiæ causam esse credimus hoc est (per scholium propositionis 13 hujus) si id quod odio habemus, destrui imaginamur, lætabimur (per propositionem 20 hujus) adeoque idem (per primam hujus partem) conabimur destruere sive (per propositionem 13 hujus) a nobis amovere ne ipsum ut præsens contemplemur, quod erat secundum. Ergo id omne quod ad lætitiam etc. Q.E.D. 

What we imagine to lead to joy, we try as much as we can to imagine (by IIIP12), that is (by IIP17) that we will try as much as we can to contemplate it as present or as actually existing. But the effort of the mind or power in thinking is equal to and at the same in nature with the effort of the body or power in acting (as clearly followed from IIP7C and IIP11C): thus as it exists absolutely we try or (what by IIIP9S is the same) we desire and intend; which was first. Next if that which is what we believe to be the cause of sadness, that is (by IIIP13S) if what we have with hatred, we imagine to be destroyed, we will be overjoyed (by IIIP20) to such a degree the same (by III first part) we will try to destroy or (by IIIP13) remove from ourselves so that we do not contemplate (it) as present, which was following. Thus everything which to joy, etc.