Allah begins to prove the truth of prophethood after He stated that there is no deity worthy of worship except Him. Allah said to the disbelievers,

﴿وَإِن كُنتُمْ فِى رَيْبٍ مِّمَّا نَزَّلْنَا عَلَى عَبْدِنَا﴾

(And if you (Arab pagans, Jews, and Christians) are in doubt concerning that which We have sent down (i.e. the Qur'an) to Our servant) meaning, Muhammad ,

﴿فَأْتُواْ بِسُورَةٍ﴾

(then produce a Surah (chapter)) meaning, similar to what he brought to you. Hence, if you claim that what he was sent with did not come from Allah, then produce something similar to what he has brought to you, using the help of anyone you wish instead of Allah. However, you will not be able to succeed in this quest. Ibn `Abbas said that,

﴿شُهَدَآءَكُمُ﴾

(your witnesses) means "Aids.'' Also, As-Suddi reported that Abu Malik said the Ayah means, "Your partners, meaning, some other people to help you in that. Meaning then go and seek the help of your deities to support and aid you.'' Also, Mujahid said that,

﴿وَادْعُواْ شُهَدَآءَكُم﴾

(and call your witnesses) means, "People, meaning, wise and eloquent men who will provide the testimony that you seek.''