A fascinating set of words, as they are in common use all the time, have changed very little since their earliest recorded usage, but we seldom differentiate clearly between them, and it is our loss. There are different types and different scales of mental competence, and different people (and therefore for writers, different characters) have differing degrees of competence in each of this group of skills or characteristics.
I'm going to start with what Webster, Wiktionary and the most TU'd entry in Urban Dictionary have to say about the modern meanings, starting with today's offering:
(1) : the ability to learn or understand or to deal with new or trying situations : reason; also : the skilled use of reason
(2) : the ability to apply knowledge to manipulate one's environment or to think abstractly as measured by objective criteria (as tests).
Capacity of mind, especially to understand principles, truths, facts or meanings, acquire knowledge, and apply it to practice; the ability to learn and comprehend.
Something severely lacking on Urban Dictionary, for god sake at least know what you're talking about before you post a load of crap on here.
The eiffel tower was built by eskimos as a present for Jesus!11!! He then changed his name to Bob Dylan and formed an emo band!1!
No form of intelligence was used in the above statement, but you'll probably believe it because you read it on the internet.
The highly esteemed Doug Harper has this to say:
late 14c., "faculty of understanding," from [Old French] intelligence (12c.), from [Latin] intelligentia, intellegentia "understanding, power of discerning; art, skill, taste," from intelligentem (nom. intelligens) "discerning," prp. of intelligere "to understand, comprehend," from inter- "between" (see inter-) + legere "choose, pick out, read" (see lecture). Meaning superior understanding, sagacity" is from early 15c. Sense of "information, news" first recorded mid-15c., especially "secret information from spies" (1580s). Intelligence quotient first recorded 1921 (see I.Q.).
It is not always the case, but I think the key to differentiating intelligence from the others is in the etymology. Because of its evident Latinness, the word tastes newer in the mouth than solid Germanic terms like wise or canny, but you'll notice that it has hardly changed from classical Latin, and probably dates from the earliest common usage of words meaning to read.
The type of mental competence implied - indeed intended in classical Latin - is one that is learned, book-learned, but goes beyond the mere absorbing of information through the sense of inter – among, between. Intelligence is the capacity to read-into things, people, events; to see beyond or between appearance to what underlies it. Intelligence requires the capacity to learn quickly, and to observe detail. When we talk about the critical sense, about reading-between-the-lines, about subtext, we are talking about intelligence. Intelligence may be necessary in order to be subtle (another mental competence that I shall be dealing with).
Of all the types of mental competence, intelligence is the one most associated with formal learning, academia, but a lack of formal learning needn't hamper intelligence, only articulacy, and therefore, possibly, the ability to act on, or analyse, information acquired through intelligent observation.
When establishing a character, the writer should be aware that the adj. intelligent will give the reader an impression of bookishness or of effete, obscure or reserved "higher" mental ability. It is the least homely of this group of words, probably both because of its Latin character, and its close association with reading.
Tomorrow I shall take a look at clever.