a. capable of being apprehended by the mind or intellect alone
b. (in metaphysical systems such as those of Plato or Kant) denoting that metaphysical realm which is accessible to the intellect as opposed to the world of mere phenomena accessible to the senses
late 14c., "able to understand," from L. intelligibilis "that can understand, that can be understood," from intelligere (see intelligence). Sense of "capable of being understood" first recorded in English c.1600.