The problem of iconicity involves a critique of de Saussure's principle of the arbitrariness of the linguistic sign. De Saussure himself admitted that the linguistic system is relatively motivated or limited in its arbitrariness. The salient question for iconicity is similarity: both the similarity between the signifier and the signified, and similarity within the context of the linguistic system. Both Peirce's and Jakobson's definitions of iconicity point to this question of similarity. In Peirce's system, the three subtypes of icon: images, diagrams, and metaphors, all involve a relation of similarity. In Jakobson's system, with its roots in the linguistic theory of Mikołaj Kruszewski, the two axes of language are those of contiguity and similarity, corresponding to arbitrariness and iconicity; Jakobson also termed these the axes of metonymy and metaphor. The iconic nature of metaphor involves a latent similarity of parallelism toward which the metaphor points, which can be interpreted, and which is beheld as an image in the mind's eye. Thus, metaphor is both the most abstract and the most concrete instance of iconicity, and is a bridge between language and the world.