Saussure's founding claim: the linguistic sign is arbitrary. There is no natural connection between the word "tree" and the concept of a tree. The link is purely conventional.
The implications are larger than they look. If signs were motivated (iconic), you'd expect all languages to converge on similar forms. Instead, languages differ radically in their phonological systems, which proves the arbitrariness.
The partial exceptions — onomatopoeia, phonesthesia — don't disprove the rule. They're islands of iconicity in a sea of arbitrariness.