Allah (swt) says in His Book addressing the Rasool (saw):
“We did not sent you save a mercy for the peoples.” [TMQ Al-Anbiaa’: 107]
The fact that he came as a mercy for them means that he brought what is of their interest (maslahah). He (swt) says:
“O Mankind! There has come to you an exhortation from your Lord, a cure for that which is in the breasts, a guidance and a mercy, for believers.” [TMQ Yunus: 57]
And He (swt) says:
“Now has there come to you a clear proof from your Lord, a guidance and a mercy.” [TMQ Al-An’am: 157]
The guidance and mercy are either for bringing about a benefit (manfa’ah) to the people or for removing away harm (mafsadah) from them. This is the interest (maslahah). This is because the maslahah is bringing about the manafi’ (benefits) and removing away the mafasid (evils). The function of determining that a (certain) matter is maslahah or not is for the Shar’ only; for it is the one that came with the maslahah, and it is the one which defines this maslahah for mankind. This is because what is meant by the maslahah is the maslahah of man as a human being. Even what is meant by the maslahah of the individual is his maslahah as a human being and not only as a particular individual. However, the maslahah is either decided by the mind (‘aql) or the Shar’. If its consideration was left to the ‘aql, the people will be unable to define the true maslahah. This is because the ‘aql is limited, so it can’t encompass the nature and reality of man, so it would not be able to decide what is of maslahah to him, for it did not comprehend the reality of man in order to decide whether this matter is of maslahah or mafsadah (evil) to him. It is only the Creator of man who understands the reality of man. Thus, no one other than the Creator of man, Allah (swt), that can certainly decide what is maslahah and what is mafsadah (evil) for him. It is true that man can think that matter is good or bad (maslahah or mafsadah) for him, but he can’t be sure of that. So leaving the decision of the maslahah to the speculation (dhann) would lead to fall in dangers. This is because he might think of a matter to be maslahah, then it appears to be mafsadah. In such a case he decided the mafsadah (bad) for man as being maslahah, thus causing harm to him. A matter could also appear to be mafsadah, then it is discovered to be maslahah. So he would remove a maslahah from man, thinking it is mafsadah, thus causing harm to him by depriving him of a maslahah. Moreover, ‘aql might judge on a matter to be maslahah, today, then man himself discovers tomorrow that thing is mafsadah, so he judges on it to be mafsadah. The same thing could occur to the mafsadah also, where he says about a thing to be mafsadah today, then he himself discovers tomorrow that it is maslahah and thus says it is maslahah. Thus, the thing itself becomes maslahah and mafsadah, a matter that is not allowed, nor it can be so. For, a matter is either maslahah or mafsadah in the same case. Leaving the decision to the ‘aql, the maslahah becomes relative and not real.
Therefore, ‘aql must not be left to decide what is the maslahah. This must rather be left to the Shar’ alone to decide,for it is the one that decides the real maslahah and the real mafsadah. The ‘aql only understands the reality (waaqi’) of the matter in a perfect way, the divine text that came about that matter, then it applies the text on the reality. If it is applied to it, then it would be a maslahah or a mafsadah, according to the text of the Shar’. If it did not apply to it, then the meaning that applies to the reality has to be sought for, so as the maslahah that Shar’ decided can be known, through the knowledge of the hukm of Allah (swt) in that matter.
Thus, maslahah is a shar’i one and not ‘aqli. It goes along with the Shar’. Thus, the maslahah exists wherever the Shar’ exists, because the Shar’ decides masalih of men.