In the political sciences all that the [purportedly Islamic--ES] philosophers have to say comes down to administrative maxims concerned with secular affairs and the government of rulers. They simply took these over from the scriptures revealed to the prophets by God Most High and from the maxims handed down from the predecessors of the prophets. Al-Ghazali” "Deliverance from Error" translated by Richard McCarthy (49).
The quoted passage occurs in an attack by Al-Ghazali on Islamic philosophy as developed, especially, by Al-Farabi and Ibn-Sina (or Avicenna) [see sections 46-48]. At first sight the key charge against them is over metaphysics, where "most of the errors are to be found," including the three bits of (proto-Spinozistic) unbelief that violate core commitments of Islam ((i) denial of bodily resurrection (while allowing spiritual resurrection), (ii) the denial of creation and the embrace of eternity of the word, and (iii) the claim that God only knows universals not individuals).
When it comes to political science, the main charge against the Islamic philosophers is that they are unoriginal and that they focus on mundane matters like public administration. I hasten to add -- in defense of all of us --that lacking in originality is compatible with truth, of course.
In fact, according to Al-Ghazali, the political science of Al-Farabi and the philosophical tradition he inaugurates is to be found in the Quran. That is to say, according to Al-Ghalazi this tradition is indeed Islamic. So, unlike the philosophers' metaphysics, their political science is true. Of course, 'Islamic' has to be seen in a wider sense than just the Quran proper but as referring to the other monotheistic, holy books (note the plural in "scriptures" and "prophets"). This fits with a passage from the Quran that I have discussed before, "We have believed in Allah and in what was revealed to us and what was revealed to Abraham, Ishmael, Isaac, Jacob, and the Descendants, and in what was given to Moses and Jesus and to the prophets from their Lord. We make no distinction between any of them, and we are Muslims [submitting] to Him." (3.84) So, one can be Muslim in some non-trivial sense, without the benefit of revelation or even the revelation of Quran (see also 3.95). At 3.3, Tora and Christian Gospel are assimilated to the Quran (see also 2.87), and this has important consequences for the Quranic vision of religious pluralism (recall).
While from a post-Enlightenment, Christian perspective it is, perhaps, odd to see all scriptures be a source of political science, it is familiar to Spinozists and readers of the Hebrew Bible (recall my posts initiated by Hazony's book on the Hebrew Bible). So, there is nothing odd from an Islamic perspective about Al-Ghazali's claim that the scriptural tradition is a source of political science. (Later in the Deliverance, Al-Ghazali gives, en passant, an example of this: “Whoever aids an unjust man, God gives the latter dominion over him,” (117; I have not found the scriptural source)).
Does it follow then that according to Al-Ghazali, the Islamic philosophical tradition originating with Al-Farabi is dispensable to a Muslim when it comes to political science? At first sight, the answer would be, yes. But Al-Ghazali also admits the philosophers have "greater refinement" in reasoning and logical acumen (43), and he is explicit in not rejecting their logical (and mathematical) achievements. It seems to follow then that the political science presented by the philosophers is more rigorously expressed than the very same science found in scriptural sources. It is not unlikely that such rigor can be beneficial for at least some purposes.So, I close with a speculative thought.
For, as I noted (while drawing on Stephen Menn), Al-Ghazali gives us enough hints in the Deliverance that he does feel entirely at liberty to state his own political science. If by Al-Ghazali's light's Al-Farabi's political science is true, it also entails, surprisingly enough, that if one wishes to have a clear understanding of Al-Ghazali's political science, one can do worse than to read Al-Farabi.