Aptitude is how an entity is keyed to perceive and experience the weaves of The Pattern. They grant specific capabilities that provide a natural tendency of perspective and direction for character development. Each aptitude anchors the subject in ways that promote the powers and experiences that reflect the nature of the aptitude. There are 4 pillar aptitudes of The Pattern, measured by a Talent Level of 0 to 4: Divine, Kinetic, Occult, and Psychic. Each represents an affinity for powers and knowledge of a specific type and rely on Essence.
Aptitude is an advanced concept, not part of the D20 Essentials for Incarna. It empowers character significantly and upgrades their challenge rating early in their character development. It offsets some of the changes to the time scales of Short and Long rests by adding capability and survivability while maintaining the Gritty Realism. Aptitude adds a much more complex element to gaming. Some of what it accomplishes, can be accomplished by use of Elan. Aptitude definitely introduces the biggest part of Incarna Character Emphasis.
Aptitude also enables other aspects of character development – it is the gateway to traits and aspects outside the core D20 experience and acquiring powers across classes without sacrificing levels. Each Ability has one of the pillars of The Pattern associated with it. Divine (Wisdom); Kinetic (Strength, Dexterity, Constitution); Occult (Intelligence), and Psychic (Charisma). Sanity does not have a specific pillar and is generally not something that characters have affinity with.
A Closer Look: Aptitudes allow characters to sacrifice versatility (multiclassing requirements become more difficult) for flavor (increasing the uniqueness of each) and early survivability (individualized powers and traits requiring aptitude). It requires a great deal of attention to ensure the player knows all the powers available to them. Aptitude adds a lot of minute dimensions and adjustments that can be hard to track. The experience of using aptitudes may have some impact on the perception of advancement. Overall, it will emphasize story and flavor over strict level based challenge ratings. Characters will begin more powerful and the arc of advancement is shallower as foes and challenges stay within a more narrow band until much higher levels. Chasing Aptitude power focuses a character on themselves. It leaves significantly less Essence for bonds with groups, entities (ex: deity or Patron), and items. Ultimately it is the responsibility of the player to monitor and maintain these various complex aspects.