SOCRATES: Is a proposition resolvable into any part smaller than a name?(Cratylus, Plato)
HERMOGENES: No; that is the smallest.
In language there are levels of units. These are sentence, clause, phrase, word and phoneme. Here Socrates narrows down the meaningful unit to the word or name.
A phoneme in itself has no meaning as such. They are the building blocks of words. The three phonemes (and letters coincidentally in this case) which make up cat (C-A-T) have no meaning in themselves. Only when combined as a particular sequence have they been designated a particular meaning. It is not natural but agreed upon by convention.
In some ways, atoms are like phonemes. Atoms extant in the observed reality have “characteristics” unique to each element. Phonemes have characteristics given to them by language creators. Not all languages have identical number and range of sounds. Some have more. Others differentiate where another may make no such distinction. But one important thing is clear: before a unit is meaningful there are smaller units which have no meaning, but only a play of differences. Socrates (and Hermogenes) pointed this out as did Saussure two millennia later.