Thank you very much for the interesting comments you have made to this questions. I wish you could elaborate more about whether the spoken language can be arbitrary or systematic.
Arbitrariness of language is an idea developed by Ferdinand de Saussure, the original wording is "l'arbitraire du signe". It is important to keep in mind that with Saussure arbitrariness does not mean "absence of fixity", but "absence of motive". The expressive value does not influence the economy of the system.
Sometimes you can find examples which contradict the principle of arbitrariness: e.g. the old *pikkare does not give french picher, as it should be following the laws of sound shift. The onomatopoetic power counterbalances the arbitrariness and the french word is piquer. Another counter-principle is analogy, grammatical regularities are created by violation of arbitrariness. De Saussure's contemporaries attacked him in an unfair way (ad hominem) by explaining his principle of arbitrariness by his plurilingualism. The indexical and the iconic character of the linguistic sign can be considered as a kind of motivation. For instance, most plurals are longer (as words) than the corresponding singulars.: child-children. Some times the word order in expressions,such as " the king and his followers" marks a social hierarchy, this means a motivation of the sign.
There is a very interesting effect linking visual perception of shapes to auditory perception of names: the Bouba/kiki effect.
While I agree that there is no apparent motivation why a given sign might have a particular morphology, I see that there is a biological substrate, predating language, over which those signs develop. So, while there is no apparent reason why a table should be called 'table' instead of 'carpet', it appears that certain visual shapes evoke certain phonetic associations.
Thank you very much for the interesting comments you have made to this questions. I wish you could elaborate more about whether the spoken language can be arbitrary or systematic.
Your question is quite controversial. Although most of the scholars subscribe to the arbitrariness, some theories back the opposite like Bow wow, ding dong.
I believe that the 'sign', i. the meaning that you assign to a word, is considered as arbitrary (though personally I don't think so, only we, the modern linguists has forgotten the real reason). But once a meaning has been decided upon, then their use should not be arbitrary. If it still is arbitrary it is because the knowledge of the person, either lexical or syntactical, is poor. In short, I don't think Language is arbitrary though words and their users can be so by poor knowledge.
Incidentally, take the word arbitrary; it means 'deciding by one's own discretion'. That is when a word was coined some discretion was used! And how is it related to arbitrage which is the noun form of arbiter (the adjective has an r after t, the noun does not)