"In space, there's no up or down."

"In outer space, there's no up or down."
"In physics, there's nothing called decceleration."
"In relativity, time is the fourth dimension."

Your parents etc. probably taught these things to you as a child, and you might've wondered at the time why that's true. Why can't we just define a direction called "up" in space? Why can't we just define decceleration as negative acceleration (or rather, acceleration in the opposite direction as motion)? Why do we count time as the fourth dimension -- why can't, I don't know, temperature be the fourth dimension?

If you're a child and your parents aren't telling you things like this, please call child protective services immediately. These factoids are incredibly important for any human being worthy of the name to internalise -- they are a special case of the general principle of symmetry, or more specifically: stuff should be defined in terms of its behaviour.

It's not that you can't define a general up or down in space, it's that you really, really shouldn't. It would serve no purpose, and would break the symmetry of space. There is no reason you should hold a specific property of the Earth as fundamental to your study of some physical phenomena in "space". Any facts that you derive must be abstracted to work in any co-ordinate system.

In the other two examples, it's a bit more subtle, as there really are specific physical phenomena associated with decceleration (e.g. harmonic motion), and time does have some special properties distinguishing it from space. Nonetheless, the mental classification is important.

This notion is fundamental to any academic discipline. Unfortunately, it seems that there is no push towards abstraction in the social sciences -- e.g. in economics, where you see a dozen different words for "externality", and a lot of definitions seem to be on entirely social terms.