Except you're wrong. Low intelligence is perfectly defined.
The entire paragraph following this is purely subjective and uses nothing but comparatives. it is very, very FAR from 'perfectly defined'.
I'll say it again.
Precise benchmarks for what each intelligence score can do that the one below it cannot. Not 'less likely', not 'more difficult', not 'might', but CANNOT. That is what 'perfectly defined' means.
Strength is perfectly defined. See that chart of carrying capacity? Str X cannot carry as much weight as Str X+1. Unless you can provide a chart with cold, hard, statistical data like that for INT, you got nothin'.
Intelligence is inherently subjective look up the theory of multiple intelligence's and read about how imprecise the use of IQ is to measure intelligence. When you find a way to provide a cold hard statistical chart for human intelligence come back, until then, "you got nothin'"