Suppose that instead of
a beak-like, protruding mini lever-type switch (as found in some older installations) you had
a vertical slider; for controlling a dimmer, perhaps.
What would be the natural position of the slider for the brightest setting — that is, when the light is fully lit (“ON”)? What about the dimmest setting (“OFF”)? (How do the sliders e.g. in
sound mixing boards or
stage light controllers typically control whatever parameter they’re supposed to control? Everyone also has
a virtual mixer panel in their computer, controlling the volume of the various audio sources.)
Now, make the range of that slider control so short and sensitive that it begins to resemble an ON/OFF lever switch... so which position of the slider is ON and which is OFF again?
Then, to simplify the mechanics — as we don’t need that dimming function, anyway — make it back into a (protruding, beak-like)
two-position lever switch.
Then, remove the lever so that only the base (on which the lever was originally attached to) remains. (Let’s assume the base was flat and not curved.) Enlarge the base. The base, just like the lever before it, will now be slightly tilted at an angle in either one of the two possible positions... facing downwards or upwards, so it becomes
a modern rocker type light switch. Now, which angle is ON and which is OFF?
Or you could make the rocker back into a lever switch by super-gluing e.g. a small length of pencil perpendicular to it...
If not taking cues from the logic of slider-type controls, you could also think of the general logic of “up” and “down” as abstract concepts... Things have their “ups” and “downs” — “up” generally meaning something light, positive, busy; “down” being some sorry, quiet, darkish, dysfunctional state. Likewise, services (especially computer-based ones) can be “up” (online) or “down” (out of order, malfunctioning, shut down for maintenance), etc.
Then there’s also the logic of the switch “beak” pointing towards the ceiling (where the light fixture is typically located) or not pointing that way... or the face of a rocker-type switch facing the general direction of the ceiling (and the light fixture) or not facing that way.
And maybe also the (electrical) safety aspect of it perhaps being easier to flick a switch — especially a lever-type switch — downwards than upwards... either by accident or on purpose, when in haste or panic.