Fairly self-explanatory, Logic is the Skillset that involves a characters' ability to problem-solve. Being a mainly Credible affair, it's usefulness lies primarily in its ability to locate and find things in the Discourse world.
A Posteriori: An attack that can be used instantly if an enemy attempts to run from combat, or normally otherwise. If it does not destroy the enemy outright, the philosopher may trace the offending enemy with the logical trail he leaves behind him through Logic + Credibility checks after combat is over. A Posteriori may also be cast without damage, but the target will know he/she has been tagged so.
A Priori: Roll your Logic + Credibility to attempt to locate a Plot Element of your choosing relative to your position in the absence of any prior evidence or logical trail. The difficulty for this extends the farther away one is from the given Element. Once attempted by a character once, this cannot be re-rolled.
Modality: Finish Him! By pulling a trick in swapping the "possible" for the "contingent", an attack buffed by Modality may deal an extra +ATTACK damage if this results in the destruction of the offending creature (reducing it to 0 or lower hit levels). However, when used this attack is necessarily lethal.
Formal Proof: A favorite of the more visceral, this attack sends several flying logical axioms rocketing towards your opponents, allowing a philosopher to split up the damage he deals as he chooses among multiple targets. If all the axioms strike one opponent, the flurry of axioms deals an extra +ATTACK damage and changes the damage type of the attack to Coherent.