A tribesman from a hot place points at what you’re wearing. “What is that?”
“A jacket,” you say.
“What is a jacket?” he asks.
What he wants to know is the purpose for which the jacket is used, and so you tell him “It keeps me warm. It protects me from the sun. It is very fashionable.”
A computer compiling information about the world is trying to fill in gaps in knowledge. It scans you and asks “what is that?”
“A jacket,” you say.
“What is a jacket?” the computer asks.
What the computer wants to know is what it matches to most closely in its existing stored knowledge. You tell it, “It is like a trenchcoat, a sweater, a coat, or a hoodie.”
An alien artist is unfamiliar with the structure of your world. It gestures its tendrils at you and asks “what is that?”
“A jacket,” you say.
“What is a jacket?” the alien asks.
What the alien wants to know is what it is that gives rise to the jacket, what the essence of jacketness is. You tell it, “It is a bunch of pieces of fabric stitched together with some thread.”
—
These are three ways in which a word can be ‘defined’ – the role it plays in the world around it (the up-definition), synonyms (lateral-definition), and the parts which construct the thing (down-definition).
Generally speaking, up-definitions are the most commonly used and the most practical. What we want to know about an object is what we can do with it. The same is applied to concepts – Love is “the thing we have for our children or parents,” surprise is “the thing that happens at a birthday you thought everyone forgot about,” and “existence” is “all this stuff you’re looking at.”
Up-definitions is also one of those things that can ‘feel like’ a satisfactory answer when what you really need is a down-definition. Discussions about morality frequently fall into the up-definition trap, where everybody’s idea of ‘wrong’ is a strictly functional thing, and then people get into conflicts over why different functional ideas are clashing with each other.
I’ve seen a few discussions of free will that also fail to recognize down-definitions; the up definition of free will is something like ‘making decisions independently’ or ‘conscious choices’ – or lateral definitions like “agency” or “my soul.” To ask about a down-definition is to ask about the fabric and thread of free will, about what little bits that idea has been built out of. Generally the down-definition I like the best is “a specific subjective sense”.
Up-definitions are useful, but down-definitions aid in presenting a more cohesive idea of what your mind is doing when it thinks. With some concepts it’s difficult to put any down-definition into words, but paying attention to the feeling of thinking about the concepts can also suffice.
Probably all concepts we use are built out of many smaller concepts, and those built out of smaller still, and oftentimes we forget this so deeply that as soon as we identify an idea like free will, we view it and wield it as a solid unit, and our debates with others feature challenging how our solid units serve functionally in the world around us. It’s like knowing how to swordfight without any knowledge of what swords are made out of – it works just fine, but it’s not holistic, and might one day prevent advancing to an expert level.