How can one differentiate between the whispers of Satan, the whispers of the self, and sound reasoning?

Issues for which there is a clear text or on which scholars have reached a consensus, do not permit consulting the heart; rather, one must submit to Allah’s حكم (ruling). As for matters where the evidence is ambiguous and scholars differ, the scrupulous Muslim should strive to be cautious and consult his heart. If his soul is content with it, he may take it. If it wavers in his chest, he should leave it, as the Prophet (peace be upon him) said: "If something wavers in your chest, leave it," and his saying: "Righteousness is that with which the soul feels at ease and the heart is content, and sin is that with which the soul does not feel at ease and the heart is not content, even if the jurists give you a fatwa (definition)." It is not permissible to act upon a fatwa with which the enquirer's soul is not content. This applies specifically to a tranquil and sound soul. As for the soul that incites to evil, its opinion is not accepted, and what contradicts the command of the Lawgiver should not be followed. The difference between the whisperings of the soul and Satan and sound thought is discerned by the balance of the Sharia (definition); what is proven invalid by the Sharia is from the whisperings of Satan and the soul that incites to evil, and what is permissible according to the Sharia is from sound thinking.