The words – square, circle, a hundred etc. convey to the mind notions so complete in themselves, and so distinct from everything else, that we are sure when we use them we know the whole of our own meaning. It is widely different with words expressing natural objects and mixed relations. Take, for instance, IRON. Different persons attach very different ideas to this word. One who has never heard of magnetism has a widely different notion of IRON from one in the contrary predicament. The vulgar, who regard this metal as incombustible, and the chemist, who sees it burn with the utmost fury, and who has other reasons for regarding it as one of the most combustible bodies in nature; – the poet, who used it as an emblem of rigidity; and the smith and the engineer, in whose hands it is plastic; and moulded like wax into every form; – the jailer, who prizes it as an obstruction, and the electrician who sees in it only a channel of open communication . . . have all different, and all imperfect, notions of the same word. The meaning of such a term is like a rainbow – everybody sees a different one, and all maintain it to be the same.1