It is impossible for any human being upon whom divine revelation did not descend with Allah's support, to understand the Shari`a, its details, and its rulings autonomously, from the Qur'an alone. It is therefore inevitable that one must look into the Sunna which was revealed together with the Qur'an and the Sunna which the Prophet inferred from striving to understand the Qur'an, in which inferring he received divine confirmation.
One must therefore avail oneself of the Sunna [i.e. accept and make use of it] in order to understand Allah Almighty's meaning and infer the details of rulings from the Qur'an, because the Sunna is the only path to achieve that objective. If the Sunna were not probative then this would not be the case. Nor would it be correct for any of those who exert scholarly striving to look into the Sunna and avail himself of it toward that end. As a result no-one would have understood what exactly they are held responsible for, the rulings would have been disregarded, the legal liabilities (al-takalif) would have been cancelled and in vain, all of which is absurd and impossible on the part of Allah Almighty and Exalted.
What makes it clear that it is impossible for a mujtahid to rely solely on the Qur'an to extract what we mentioned, is that because the Qur'an has reached such a level of miraculous inimitability (i`jaz) and the farthest limit of eloquence and economy, it contains many secondary meanings, treasure troves and secrets, many of which are known only to Him Whose Speech this is, and to him upon whom their exposition was descended and revealed .
Al-Bukhari narrated in this respect that when the Prophet was asked about the status of donkeys (i.e. in comparison with horses in merit, especially in jihad), he replied: "Nothing was revealed to me concerning them except this all-encompassing, peerless verse: {And whoso does good an atom's weight will see it then, and whoso does ill an atom's weight will see it then} (99:7-8)."1 Consider how the Prophet took the ruling concerning donkeys from that verse, and is anyone else able to do this?2
Furthermore, the Qur'an contains stipulations that are summarized (mujmala) and others that are complex (mushkila). It is therefore necessary, in order to put them into practice, to have an explanation that expounds, clarifies, interprets, and explains them. It is further necessary that such an exposition come from Allah Almighty and Exalted because it is He that has thrust responsibility upon His servants and therefore He knows what is being meant before anyone else. And this explanation is the Sunna that revelation brought or upon which Allah Almighty and Exalted confirmed His Prophet if it originated in elucidatory striving on his part. Hence Allah Almighty and Exalted said {And We have revealed unto you the Remembrance that you may explain to mankind that which has been revealed for them} (16:44).