Propensity
Part of speech: noun
Definition: A tendency, preference, or attraction.
Example sentence: The dog commends himself to our favor by affording play to our propensity for mastery.
Aptness
Part of speech: noun
Definition: Fitness; suitableness; appropriateness; as, the aptness of things to their end.Disposition of the mind; propensity; as, the aptness of men to follow example.Quickness of apprehension; readiness in learning; docility; as, an aptness to learn is more observable in some children than in others.Proneness; tendency; as, the aptness of iron to rust.