Principles are a key mechanic of the Secret Histories universe. Every object and living being is imbued with certain Principles which give them access to certain kinds of rituals or magic effects. Though all Principles are to some degree universal and mutable based on a character or object's environment, physical/emotional state, and recent actions, everything and everyone has certain Principles to which they are especially predisposed. For characters, these natural Principles are generally influenced by their personality, skills, and recurring actions, such as those associated with their profession. A summary of all Principles can be viewed here The Librarian is capable of discerning a person or thing's inherent Principles and thus knowing in a general sense what kinds of rituals or magic they may be suited for.

Note that a person or object's Principles tend to be quite abstract and subjective, so don't worry too much about if your assessment is "accurate." For some canon examples, the somber and kind midwife, Mrs. Kille, has 2 Grail and 1 Heart, while the traveling Elegaic Poet has 4 Winter, 4 Moth, and 4 Sky. For some well-known character examples I've made up on the spot, I'd give Darth Vader 4 Edge, 4 Moon, and 2 Forge, whereas I'd give Spock 2 Lantern and 2 Sky. Really just go with whatever feels right!