>There are no humans that "can't" hear the difference between L and R.
You'd be surprised.
There are no 100% objective sounds the ear directly hears -- interpretation and identification of sounds happens at the brain, and a person that was raised with a specific language/pronunciation can be "deaf" to the difference of certain sounds.
Evidence from Best & Strange (1992) and Yamada & Tohkura (1992) suggest that Japanese speakers perceive English /r/ as somewhat like the compressed-lip velar approximant /w͍/ and other studies[4] have shown speakers to hear it more as an ill-formed /ɺ/. Goto (1971) reports that native speakers of Japanese who have learned English as adults have difficulty perceiving the acoustic differences between English /r/ and /l/, even if the speakers are comfortable with conversational English, have lived in an English-speaking country for extended periods, and can articulate the two sounds when speaking English. Japanese speakers can, however, perceive the difference between English /r/ and /l/ when these sounds are not mentally processed as speech sounds. Miyawaki et al. (1975) found that Japanese speakers could distinguish /r/ and /l/ just as well as native English speakers if the sounds were acoustically manipulated in a way that made them sound less like speech (by removal of all acoustic information except the F3 component). Lively et al. (1994) found that speakers' ability to distinguish between the two sounds depended on where the sound occurred. Word-final /l/ and /r/ with a preceding vowel were distinguished the best, followed by word-initial /r/ and /l/. Those that occurred in initial consonant clusters or between vowels were the most difficult to distinguish accurately. Bradlow et al. (1997) provide evidence that there is a link between perception and production to the extent that perceptual learning generally transferred to improved production. However, there may be little correlation between degrees of learning in perception and production after training in perception, due to the wide range of individual variation in learning strategies.
The same can happen with colors (for them, depending on whether the language/culture has a name for them).
So what you write is not 100% true. At best, it's debatable:
You'd be surprised.
There are no 100% objective sounds the ear directly hears -- interpretation and identification of sounds happens at the brain, and a person that was raised with a specific language/pronunciation can be "deaf" to the difference of certain sounds.
Evidence from Best & Strange (1992) and Yamada & Tohkura (1992) suggest that Japanese speakers perceive English /r/ as somewhat like the compressed-lip velar approximant /w͍/ and other studies[4] have shown speakers to hear it more as an ill-formed /ɺ/. Goto (1971) reports that native speakers of Japanese who have learned English as adults have difficulty perceiving the acoustic differences between English /r/ and /l/, even if the speakers are comfortable with conversational English, have lived in an English-speaking country for extended periods, and can articulate the two sounds when speaking English. Japanese speakers can, however, perceive the difference between English /r/ and /l/ when these sounds are not mentally processed as speech sounds. Miyawaki et al. (1975) found that Japanese speakers could distinguish /r/ and /l/ just as well as native English speakers if the sounds were acoustically manipulated in a way that made them sound less like speech (by removal of all acoustic information except the F3 component). Lively et al. (1994) found that speakers' ability to distinguish between the two sounds depended on where the sound occurred. Word-final /l/ and /r/ with a preceding vowel were distinguished the best, followed by word-initial /r/ and /l/. Those that occurred in initial consonant clusters or between vowels were the most difficult to distinguish accurately. Bradlow et al. (1997) provide evidence that there is a link between perception and production to the extent that perceptual learning generally transferred to improved production. However, there may be little correlation between degrees of learning in perception and production after training in perception, due to the wide range of individual variation in learning strategies.
The same can happen with colors (for them, depending on whether the language/culture has a name for them).
So what you write is not 100% true. At best, it's debatable:
https://en.wikipedia.org/wiki/Perception_of_English_/r/_and_... l/_by_Japanese_speakers
https://en.wikipedia.org/wiki/Linguistic_relativity_and_the_...
https://eagereyes.org/blog/2011/you-only-see-colors-you-can-...
https://en.wikipedia.org/wiki/Blue%E2%80%93green_distinction...
https://www.businessinsider.com.au/what-is-blue-and-how-do-w...