Illusion is an intellectual knowledge (buddhijanya). Intellectual knowledge is vikalpi (uncertain, with wrong belief of the self). Whereas, the Real Knowledge (Gnan) is nirvikalpi (Certain, free from I-ness & My-ness). There is no feeling of differences in it. Therefore one is to only understand the real knowledge.
Aptasutra ( Spiritual Quotes by Param Pujya Dadashri ) # 382