 Tusi mentions two types of argumentation that the tradition describes as assertoric proof or quia (burhan inni) and demonstrative proof or propter quid (burhan limmi), the former is an inference from effect to cause and the latter is from cause to effect. The demonstrative proof for Mulla Sadra is the one that is ‘most reliable, most illuminating and most noble’ and it involves an inference of reality by taking God, Existence, as a witness to the totality of existence. This is what he calls the ‘way of the veracious’ (sabil al-siddiqin). What he means by siddiq is not the same as Avicenna and the gap in meaning is a good illustration of the difference in their philosophical method. For Mulla Sadra, the siddiq is one who possesses intuition and inner disclosure that is attained through grace and spiritual exercise. In the exegesis of Q. 57:19 on the phrase ‘those who believe in God and His messengers are the veracious ones and witness before their Lord’, he argues that the siddiq is characterised by witnessing the truth through inner revelation: