I have particularly noticed the change in his mental attitude that became apparent at Berneval, because I think that it throws light on the character of the work he did after leaving prison, so markedly different from that of _De Profundis_, or _Intentions_, or _The Sphinx_, or any other of the delightful designs it had pleased him to embroider. What is remarkable in _The Ballad of Reading Gaol_, apart from its strength, or its violence of emotion, is a change in the quality of Wilde's language. A distinction between decoration and realism, though it immediately suggests itself, is too blunt to enable us to state clearly a change in Wilde's writing that it is impossible to overlook. We require a more sensitive instrument, and must seek it in a definition of literature, a formula that is concerned with the actual medium that literature employs. To make such a definition I have borrowed two words from the terminology of physical science. Energy is described by physicists as kinetic and potential. Kinetic energy is force actually exerted. Potential energy is force that a body is in a position to exert. Applying these terms to language, without attempting too strict an analogy, I wish to define the medium of literature as a combination of kinetic with potential speech. There is no such thing in literature as speech purely kinetic or purely potential. Purely kinetic speech is prose, not good prose, not literature, but colourless prose, prose without atmosphere, the sort of prose that M. Jourdain discovered he had been speaking all his life. It says things. An example of purely potential speech may be found in music. I do not think it can be made with words, though we can give our minds a taste of it in listening to a meaningless but narcotic incantation, or a poem in a language that we do not understand. The proportion between kinetic and potential speech and the energy of the combination vary with different poems and with the poetry of different ages. Let me take an example of fine poetry, and show that it does perform in itself this dual function of language. Let us examine the first stanza of Blake's "The Tiger":-- "Tiger! Tiger! burning bright In the forests of the night, What immortal hand or eye Could frame thy fearful symmetry?" It is impossible to deny the power of suggestion wielded by those four lines, a power utterly disproportionate to what is actually said. The kinetic base of that stanza is only the proposition to a supposed tiger of a difficult problem in metaphysics. But above, below, and on either side of that question, completely enveloping it, is the phosphorescence of another speech, that we cannot so easily overhear.[10]