Ukweli ni lengo la akili katika kujua mambo yote kwa dhati iwezekanayo. Unaweza kujulikana kuanzia hisi kwa kufikiria, lakini pia kwa kushika imani ya dini fulani inayosadikiwa imefunuliwa na Mungu. Wazo hilo ni la msingi hasa katika Ukristo, na kwa namna ya pekee katika Injili ya Yohane. Aristotle, "The Categories", Harold P. Cooke (trans.), pp. 1–109 in Aristotle, Volume 1, Loeb Classical Library, William Heinemann, London, UK, 1938. Aristotle, "On Interpretation", Harold P. Cooke (trans.), pp. 111–179 in Aristotle, Volume 1, Loeb Classical Library, William Heinemann, London, UK, 1938. Aristotle, "Prior Analytics", Hugh Tredennick (trans.), pp. 181–531 in Aristotle, Volume 1, Loeb Classical Library, William Heinemann, London, UK, 1938. Aristotle, "On the Soul" (De Anima), W. S. Hett (trans.), pp. 1–203 in Aristotle, Volume 8, Loeb Classical Library, William Heinemann, London, UK, 1936.