The 'aqeedah has two fundamental sources, which are: -
1 - The Book of Allaah - the Most High (the Noble Qur'aan).
2 - What is authentic from Allaah's Messenger sallallaahu ‘alayhi wa sallam. Since the Messenger sallallaahu ‘alayhi wa sallam does not speak from his desires, rather his words are Divine Revelation sent down to him.
And the consensus (Ijmaal of the Pious Predecessors (salaf-us-saalih); a source based upon the Book and the Sunnah. [Refer to Al-I'tisaam (2252) of ash-Shaatibee].
As regards the natural state (fitrah) and the sound intellect ('aql) then they will agree with and conform to the Book and the Sunnah and will be able to arrive at the general principles of 'aqeedah - but not its details. So the 'aql and fitrah affirm - in general - Allaah's existence and that He is Most Great, that it is essential to obey and to worship Him; and that He has the Attributes of Greatness and Perfection. Likewise, the sound and correct 'aql and fitrah realise the necessity of there being Prophets, the sending of Messengers and that there has to be Resurrection and requital of actions; but only in general, not in detail. However, as for other matters and the rest of the matters of the Unseen, then there is no way to find them out in detail, except by way of the Book and the Sunnah, otherwise it would not be from the Unseen.
And contradiction between a clear text of the Book and the Sunnah and between the sound 'aql cannot be imagined and is infact an impossibility. If there appears to be a contradiction between the two, then the Revelation is given precedence and is considered decisive [Consult Sharhul-Aqeedatut-Tahaawiyyah (pp.140-141) of Ibn Abil-'Izz al-Hanafee. Also refer to Dar' Ta'aarudil-Aql ma'an-Naql I/88-280) of Ibn Taymiyyah. Indeed, refer to the whole book, as it is very beneficial in this matter], since it comes from the one who is infallible, sallallaau 'alayhi wa sallam whereas the 'aql of a person is not infallible. Indeed, the 'aql is the deficient perception of humans; it is open to misconception, error, forgetfulness, desires, ignorance and inability. Thus it is something that is certainly deficient.