The goal of dharma is universal welfare. The great men who produced the works on Dharmasastra didn't have a trace of self-interest in them and had nothing but the thought of the happiness of all creatures. These treatises are the authority on which dharma is founded. You find the form of things, the image, with your eyes; you perceive sound with your ears; you know dharma with the help of Dharmasastra.
The Vedas (Sruti) are the root of all dharma. After Sruti comes Smrti. The latter consists of the "notes" based on Smrti. It is the same as Dharmasastra. Another guide for the dharma is the example of great men. The Puranas provide an answer to how great men conducted themselves. Then there is sistacara to guide us, the life of virtuous people of noble character. Not everybody's conduct can be a guide to us. The individual whose life is an example for the practice of dharma must have faith in the sastras and must live in accordance with their ordinances. Besides, he must be free from desire and anger. The conduct of such men is sistacara. Another authority or guide is what we know through our conscience in a state of transparency.
In matters of the Self, of dharma and religion, the Vedas are in the forefront as our guide. Next come the dharmasastras. Third is the conduct of the great sages of the past. Fourth is the example of the virtuous people of our own times. Conscience comes last in determining dharma.
Now everything has become topsy-turvy. People give importance first to their conscience and last to the Vedas. We must consult our conscience only as a last resort when we have no other means of knowing what is dharma with reference to our actions. Why is conscience called one's "manahsaksi"? Conscience is fit to be only a witness(saksi), not to be a judge. A witness often gives false evidence. The mind, however, doesn't tell an untruth - indeed it knows the truth of all things. " There is no deceit that is hidden from the heart(mind), " says Auvvai. Conscience may be regarded as a witness. But nowadays it is brought in as a judge also in dharmic matters. As a witness it will give us a true report of what it sees or has seen. But on the basis of it we cannot give on what is just with any degree of finality. "What I think is right, " everybody would try to satisfy himself thus about his actions if he were to be guided only by his conscience. How can this be justified as the verdict of dharma?
We often hear people say, "I will act according to what my conscience tells me. " This is not a right attitude. All at once your conscience cannot be given the place of a judge. It is only when there is no other way open to you that you may tell your mind: "You have seen everything as a witness. Now tell me your opinion. " The mind belongs to each one of us as individuals. So it cannot be detached from our selfish interests. The place it has in one's personal affairs cannot be given to it in matters of religion. On questions of dharma the opinion of sages alone is valid, sages who were concerned with universal welfare and who transcended the state of the individual concerned with his own mind [or with himself].
For a general background, please see here