 Also some concepts are derived from others; the word for the derived concept should be grammatically derived from the word for the other concept. The words for underived concepts should not be grammatically derived. Al-Fārābī notes that in practice this correlation sometimes fails. For example, he takes it that the participle “living” (ḥayy) is derived, but it is sometimes used to signify the underived concept “animal” ([Letters] (26) 74.16, (36) 81.18f). Likewise the underived concept “is” is sometimes signified by the derived passive participle mawjūd ([Letters] (84) 113.9).