Imagine that your car breaks down and you bring it to a mechanic and he tells you, “Oh, your car has a case of broken-downness.” You’d know right away: this guy has no idea what he’s talking about. “Broken-downness” is an abstraction; it doesn’t refer to anything, and it’s not going to help you fix a car.
  
  Instead, a good mechanic will describe your car’s problem in terms of entities and rules. “Your spark plugs are shot [ENTITIES], so they can’t make the pistons [ENTITIES] go up and down anymore [RULES].” 
  
  It’s easy to see how ridiculous abstractions are when we’re talking about cars, but it can be surprisingly hard to notice them when we’re talking about science.
  
  For instance, if you feel sad all the time, a psychologist will probably tell you that you have “depression.” But depression is an abstraction — it involves no theory of the entities or rules that cause you to feel sad. It’s exactly like saying that your car has “broken-downness.” Abstractions like this are basically useless for solving problems, so it’s not surprising that we aren’t very good at treating “depression.”