Magical strength is the level of mystical energy residing within and around the spell caster, which can be used to power his or her spells. This amount of energy grows as the caster gains experience and ability (ie. levels). This energy is drained away by the casting of spells, but returns with proper sleep (8 hours of sleep leads to 100% MSTR recovery, less sleep leads to proportionally less recovery).

Magical potential is the caster’s ability to tap into his or her available energy. This potential comes from the caster’s self-confidence and memory of the forces of magic. MPOT is lost with the casting of spells, at exactly the same time as the MSTR to power the spells. However, magical potential does not return automatically with rest. To regain potential, the caster must study his spellbooks (if a wizard) or pray to his diety (if a priest, paladin, or ranger) for at least 10 minutes per point.

Since the passing of time also affects memory and the caster’s confidence in his memory, each day without study or prayer causes the loss of one point of potential. This one point can be retained by studying for a mere 10 minutes. This same 10 minutes of study can lead to the recovery of one point of potential, so a first level mage only has to study for ten minutes a day (and unless a specialist, additional study will not help).

Wizards are only able to cast spells which they have studied in the last week. Priests, rangers, and paladins can only cast spells that they prayed for knowledge about. If the wizard loses his spellbooks, he loses access to those spells!

A couple of examples:

Talos (a 9th level mage) uses up all of his spell casting ability. He gets a good night of sleep, then wakes and studies his spellbooks for 3 hours. His MSTR has returned to 32 (due to good sleep) and his MPOT has risen to 18 (due to his studying). Since his MPOT is only 18, he can only cast 18 levels of spells until he studies more.

Talos again casts several spells (totalling 14 levels). He gets another night of good rest, raising his MSTR to 32 again. He still has MPOT of 4, but unless he studies soon, he’ll lose 1 point. He studies for 4 hours, retaining all 4 points and gaining back 24 more, for a total MPOT of 28. He can cast 28 levels of spells.

Specialist wizards gain extra magical strength and magical potential, matching their additional casting ability. This extra power must be used to cast spells from within their chosen school of magic. Thus, a 2nd level illusionist would have MSTR of 4 and MPOT of 4, but 2 points of each would have to be used for an illusion spell. It’s best to keep separate totals of MSTR and MPOT for specialists.

Paladins function like priests. They have to pray for knowledge about their spells.

Rangers function like priests. They have to pray for knowledge about their spells.

Multi-classed spellcasters (mage/clerics, for example) keep separate totals of MSTR and MPOT for each class. Priestly energies and confidence do not translate to wizardly energies and confidence under this system.

Bards are the only exception to these rules. They use music and/or song to channel the magical energy naturally found around them into spell effects. Since they do not hold this energy near them, they do not have an unusual magical aura (and no MSTR score). However, the stresses of tuning this energy does wear on their mental abilities, so they do have a MPOT score (and a limit to the amount of magic they can tune), which is double that derived from the amount of magic listed in the Player's Handbook. A bard's MPOT returns with proper rest (just as a wizard's MSTR returns).

Keep in mind that material components are still required for all spells. If a wizard has studied fireball, has enough MSTR and MPOT, but does not have any bat guano, he cannot cast the spell. The same goes for priests.