 Al-Fārābī defines a concept to be “true” (ṣādiq) if it is the same (bi-‘aynihi) in the external world as it is in the soul ([Letters] (88) 116.3f) For this definition to make sense, the concept need not be propositional. For example ([Letters] 118.5f), the concept “vacuum” is not true, because it exists only in the mind and not in the external world (Abed [1991: 111–115] discusses the relationship between existence and truth in al-Fārābī).