Astrologers think that astrology, with the knowledge it gives of astral powers, individually or in combination, and of astral influences upon elemental creations, enables them to know the things that are going to be in the world of the elements, before they are created. The positions of the spheres and the stars are thus (taken to) indicate every single kind of future event, both universal and individual.
The ancient (astrologers) were of the opinion that the knowledge of astral powers and influence is acquired through experience. It is something that all human lives combined would not be able to achieve, because experience is obtained through numerous repetitions which make the attainment of (empirical) knowledge or conjectures possible. Astral revolutions may be very long. Greatly extended periods of time are required for their repetition. Even all the lives in the world combined would be too short for (observing) them.
Some weak-minded (astrologers) take the view that the knowledge of astral powers and influences comes through revelation. This is a fallacy. They themselves have furnished us with arguments sufficient to refute it. The clearest proof is that, as one knows, of all people, the prophets are least familiar with the crafts. They do not undertake to give information about the supernatural, unless it comes to them from God. Why, then, should they claim to produce (supernatural information) through a craft (such as astrology) and make it the law for their followers to do so?
Ptolemy and his followers were of the opinion that the stars are able to indicate the future as the natural result of a temper they produce in the elemental existing things. He said:
"The activity of sun and moon and their influence upon elemental things are so obvious that no one can deny them. For instance, the sun influences the changes and tempers of the seasons, the ripening of fruits and grains, and so on. The moon influences humidity, the water, the process of putrefaction in substances and cucumbers, and so on."
Ptolemy continued:
"With regard to the stars that come after sun and moon, we have two approaches. One - which, however, is unsatisfactory - is to follow the tradition of the astrological authorities. The other is to rely upon conjecture and empirical knowledge gained through comparing each star to the sun, whose nature and influence is clearly known to us. We thus note whether a given star increases the power and temper of the sun at its conjunction with it. If this is the case, we know that the nature of that particular star agrees with that of the sun. If, on the other hand, the star diminishes the power and temper of the sun, we know that its nature is opposite to that of the sun. Then, when we know the individual powers of the stars, we can also know them in combination. That happens when they look upon each other in the franc, the quarters, or other aspects. The knowledge here is derived from the natures of the signs of the zodiac, which similarly are known through comparison with the sun."
"Thus, we get to know all the astral powers. They exercise an influence upon the air. This is obvious. The resulting temper of the air communicates itself to the created things below the air, and shapes sperm and seeds. Thus, this temper comes to underlie the body created from sperm or seed, the soul which attaches itself to the body, pours itself into the body, and acquires its perfection from the body, and all the conditions depending on soul and body. The qualities of sperm and seed are the qualities of the things that are created and produced from sperm and seed."
Ptolemy continued:
"Still, astrology remains conjectural and is not certain in any respect. It also forms no part of the divine decree - that is, predestination. It is just one of the natural causes common to all existing things, whereas the divine decree is prior to everything."
This is the sum total of the discussion by Ptolemy and his colleagues. It is found in the Quadripartitum and other works.
It makes the weakness of the achievements of astrology clear. Knowledge of, or conjectures about, things that come into being can only result from knowledge of all their causes, that is, agent, recipient, form, and end. According to (the astrologers), the astral powers are merely agents. The elemental part is the recipient. Furthermore, the astral powers are not the sole agents. There are other powers that act together with (the astral powers) upon the material element (involved), such as the generative power of father and species contained in the sperm, the powers of the particular quality distinguishing each variety within the species, and other things. When the astral powers reach perfection and are known, they still are only one among many causes that go into the making of a thing that comes into being.
Furthermore, in addition to a knowledge of astral powers and influences, a great amount of conjecturing and guesswork is required. Only then is (the astrologer) able to guess that a thing might happen. Now, conjecturing and guesswork are powers in the mind of the student. They are not causes or reasons of the things that come into being. Without conjectures and guesswork, (astrology) steps down from conjecture to doubtfulness.
Such is the situation even if one's knowledge of the astral powers is accurate and without defect. Now, that is difficult. The ability to calculate the courses of the stars is required in order to know their positions. Moreover, it is not proven that every star has its own particular power. The method Ptolemy used in establishing the powers of the five planets, that is, comparison with the sun, is a weak one, because the power of the sun is superior to all other astral powers and dominates them. Thus, one hardly ever becomes aware of an increase or decrease in the (powers of the sun) at its conjunction (with a given star), as Ptolemy said. All this speaks against the assumption that it is possible to predict things that will happen in the world of the elements with the help of astrology.
Furthermore, it is wrong to assume that the stars exercise an influence on the world below them. It has been proven that there is no agent but God. In this connection, speculative theologians use the self-evident argument that how causes are related to the things caused is not known, and suspicion attaches to the conclusions of the intellect regarding what appears superficially to be (due to some definite) influence. Perhaps, the relationship of (the causes to the things caused) is effected by some other than the ordinary form of influence. The divine power (would seem to) tie the two together, as it does with all created things, both high and low, especially since the religious law attributes all happenings to the power of God and does not want to have anything to do with anything else.
Prophecy also denies the importance and influence of the stars. For instance, (Muhammad) said:
"No eclipse of either sun or moon takes place to indicate the death or life of anybody."
Thus, the worthlessness of astrology from the point of view of the religious law, as well as the weakness of its achievements from the rational point of view, are evident. In addition, astrology does harm to human civilization. It hurts the faith of the common people when an astrological judgment occasionally happens to come true in some unexplainable and unverifiable manner. Ignorant people are taken in by that and suppose that all the other (astrological) judgments must be true, which is not the case. Thus, they are led to attribute things to some being other than their Creator.
Further, astrology often produces the expectation that signs of crisis will appear in a dynasty. This encourages the enemies and rivals of the dynasty to attack it and revolt against it. We have (personally) observed much of the sort. It is, therefore, necessary that astrology be forbidden to all civilized people, because it may cause harm to religion and dynasty. It is our duty to try to acquire goodness with the help of the things that cause it, and to avoid the causes of evil and harm. That is what those who realize the corruption and harmfulness of this science must do.
This situation should make one realize that even if astrology were in itself sound, no Muslim could acquire the knowledge and habit of it. He who studies it and thinks that he knows it fully is most ignorant of the actual situation. Those who are eager to learn it - and they are very, very few-have to read the books and treatises on astrology in a secluded corner of their houses. They have to hide from the people and are under the watchful eye of the great mass. And then, astrology is a very complicated subject with many branches and subdivisions and is difficult to understand. How could people under such conditions acquire a mastery of it? Jurisprudence is of general usefulness in both religious and worldly affairs; its sources are easily available in the Qur'an and the accepted Sunnah, and it has been studied and taught by the great mass of Muslims. There have been classes and seminars on jurisprudence. There has been much instruction in it and a great many lectures. Still, only an occasional individual in each age and generation (race) has been able to master it. How, then, can anyone learn a subject (such as astrology) that is discarded by the religious law, banned as forbidden and illegal, concealed from the great mass, its sources difficult of access, and that, after the study and acquisition of its basic principles and details, requires a great amount of support from conjecture and guesswork on the part of the student? How could anyone become skilled in such a subject in the face of all (these difficulties)? When all this is taken into consideration, the soundness of our opinion (with regard to astrology) will become clear ...
From Al-Muqaddimah (Introduction to History), translated from the Arabic by F. Rosenthal, edited and abridged by N. J. Dawood, 1967 Princeton University Press.