I am not sure there’s a single thing called “intelligence.”
Obviously, people have different mental aptitudes and capabilities! I want to put that right up front because people tend to get confused and assume that if one says “I’m not sure intelligence is a singular thing” one actually means “…because no one has different cognitive abilities at all!” However, I think that there are a lot of different kinds of cognitive abilities under the thing normally called “intelligence,” some of which are correlated and some of which are not, and that classifying them all as “intelligence” risks conflating them.
So here are some things I’ve thought of that people mean when they say “intelligence”:
Sounding smart. Your ability to say things like “when I read the Aeneid in the original Latin” and “reifying the social construction of homonormative sexuality” and “its transcendence degree over Q, the prime field of C, is the cardinality of the continuum.” If you want to sound smart, I recommend studying mathematics, philosophy, physics, or classics, using lots of jargon and complex sentence structure, and adopting an aura of arrogance and contempt for the plebes. (Do not, however, brag about your IQ or your Mensa membership. People will laugh at you.)
Unfortunately, while sounding smart does correlate to some degree to Actually Meaningful Cognitive Abilities, a lot of times the person talking about how Feynman is a classy-ass motherfucker has way more knowledge of physics than the person wittering about quantum vibration.
Credentialing. Having a high school degree, a college degree, a master’s, or a PhD. Of course, a lot of people with a college degree are smart. On the other hand, a lot of them got drunk and went to football games and played video games for four years and passed their classes because they cheated or took nothing but easy courses. And a lot of people who don’t have a college degree are very intelligent but were too poor to attend college, flunked out because of depression, or didn’t even realize college was an option for them. Even attendance at a top college is not necessarily an indicator of Actually Meaningful Cognitive Abilities, given the rate of legacy admissions to Ivy League schools. Class and ability are incredibly important here: poor people and disabled people are, all things considered, less likely to have access to credentials.
Knowledge. Knowing Lots of Things. Knowing lots of things about lots of topics, particularly topics that are generally considered academic, is often considered to be a sign of intelligence. Note that there are lots of autodidacts who know lots of things, and people with impressive credentials who forgot it all or never even learned. Also that knowing things does not necessarily mean that you know what you should do with that knowledge, or that you can come up with brilliant new ideas on your own.
Memory. Or, well, semantic memory, anyway. The ability to remember all the world capitals, not the ability to remember where you put your car keys. This is linked to knowledge, but different– after all, if you have a good semantic memory but little library/Internet/school access or no desire to learn things, you’re probably not going to know a whole hell of a lot.
Reasoning ability. Your ability to do things like solve math problems and do well on tests. This gets called “intelligence” a lot, and I don’t have a problem with people calling this “intelligence” as long as they’re careful with their usage. Because, once again, you can have a lot of reasoning ability and be irrational or have really massively incorrect opinions or drop out of high school or speak solely in Tumblr speak.
Creativity: The ability to think of new ideas! I… really can’t think of much to say about this, okay. But it’s a thing.
Executive functioning. Executive functioning is one’s brain’s ability to do things like “remember what you’re doing” and “do what you’re supposed to do when you’re supposed to do it” and “get to class on time.” People don’t necessarily assume that people with a low level of executive functioning are stupid– as everyone with poor executive functioning who gets told “you’re so smart, why can’t you just–?” knows– but a person with a high level of executive functioning often is likely to come off as more intelligent.
Rationality. Susceptibility to cognitive biases! Like “I already know this, so I’m going to avoid any information that might disprove it.” Or “look, the coin came up heads a dozen times, we’re due for a tail.” Or “that person cut me off because he’s a jerk, but I cut this person off because the sun was in my eyes.” (It occurs to me that not all cognitive biases have the same origin, so you might be able to divide up this category further, but I don’t know enough to say what the divisions are.)
Desire to know. Do you like thinking? Do you want to learn? Is knowing things fun to you? Do you like debate and solving problems? This is the one I tend to value most highly in people, and what I’m most likely to mean when I say “smart.” (Which is weird, because everyone else seems to mean “reasoning ability.” Harrumph. Perhaps I should start saying I only want to be friends with people with a high need for cognition instead.)
All those those things are pretty heavily correlated with each other. For instance, wanting to know things, reasoning ability, and memory are all probably correlated to how much you know. Reasoning ability, creativity, and memory are also correlated fairly well, at least if you believe our current methods of measuring those three things work fairly well. Credentialing and sounding smart are correlated to all the Actually Meaningful Cognitive Abilities.
But they are not all the same thing. Do not assume that because someone is good at reasoning that they’re necessarily rational, or that because someone has a college degree that they necessarily know lots of stuff. In particular, credentialing, sounding smart, and knowledge are influenced by things like social class and access to an actually decent educational system and not being mentally ill and having parents that take an interest in your upbringing and who your friends were as a kid and so on.