Be faithful to yourself: Polonius's precept to his son, and in Japanese a maxim in its own right. 忠実 is faithful, loyal, the word for serving with a whole heart and for a copy true to its original, which is what Shakespeare's true meant too; なれ is the old imperative, be, which makes the line a maxim rather than a sentence. The six characters are a translator's, Kume Masao's Polonius, and his readers already knew them as the watchword of a generation of Japanese writers who set faithfulness to the self above every rule and model, so a Japanese reader hears a proverb where an English reader hears the play. The words carry the precept; the half that follows it, that you cannot then be false to any man, is yours to keep.
The six characters are 久米正雄's 1922 translation of Polonius's line in Hamlet (about 1600); but the same words already existed in Japanese as a naturalist watchword for faithfulness to the self, independent of Shakespeare, from a 1908 essay, and the two origins fused only when 久米 gave Polonius the maxim his readers already knew.
We want you to be as happy with the meaning as you are with Master Takase's art. To learn more, visit Philology of 己に忠実なれ.