From Imagination and thought in the Middle Ages - by C.S. Lewis. An essay delivered in 1956; posthumously published in Studies in Medieval and Renaissance Literature, edited by Walter Hooper, 1966.
"...the planets had more than a physical effect. They influenced the course of events and they influenced human psychology. Born under Saturn, you were disposed to melancholy, born under Venus to amorousness.
"The important question, theologically, was whether the planets compelled or merely disposed men to action.
"If they compelled, then of course there was an end to human freedom and responsibility.
"If they merely disposed, then planetary influence, like heredity or health or education, was merely part of the concrete situation handed over to the individual to do the best he could with.
"The theologians were in fact, as so often, fighting against determinism.
"Nor were they fighting against a phantom: in Renaissance times, if not before them, astrological determinism was very widely accepted. It seemed (odd as this sounds to us) to have the support of age-old experience and common sense, and the theological resistance seemed idealistic wishful thinking.
"In the Middle Ages men's mind's no doubt wavered. The ordinary, moderate, respectable view was summed up in the maxim sapiens dominabitur astris; a wise man, assisted by Grace, could get over a bad horoscope just as he could get over a naturally bad temper.
"On a practical level orthodox people, while admitting planetary influence, strongly disapproved of 'judicial astrology', the lucrative practice of foretelling the future.
"They did not need to deny that some astrological predictions might be correct.
"Planetary influence could not remove free will but it could alter the states of mind and imagination which free will has to deal with.
"Any man can master the psychological raw material and thus refute the prediction; but few men do and therefore the predictions will succeed as regards the majority.
"Just in the same way and for similar reasons a modern theologian might say that Marxian predictions based on economic determinism or Freudian predictions based on psychological determinism will usually be true, and true about mass-behaviour, but not necessarily about a given individual."
Medieval thinkers were more rational about astrology that modern thinkers are about genetics.
Medieval Christian understanding was built-around free will: that free will could be influenced, but was not ultimately compelled, by emotions and images, by disposition and circumstances, by planets and angels and demons.
By contrast, the modern 'scientific' understanding, by its prior assumptions, excludes free will from rational causal explanations.
Yet the modern thinker does not recognize that he has excluded free will in his assumptions. Instead he believes that he has discovered free will does not exist.
The modern thinker ignores free will when he is at work - until after he has reached his conclusions; then he wonders how on earth he can make sense of his conclusions in a world where free will is assumed.
Having constructed a model of the world without free will (a model world with which he is well satisfied); he begins to wonder - in a bewildered fashion - how free will might somehow be re-inserted into the description of the world?
Then, finding that free will has no role, serves no necessary causal function in his model world; the modern thinker believes that somehow free will has thus been refuted, has been discovered - after empirical investigation - to be absent, been exposed as a primitive philosophical error...
Once what Lewis called 'chronological snobbery' has been pushed aside, a decline in human intelligence, in reasoned discourse, seem inescapable.
Again and again I find that modern thinking cannot distinguish between assumptions and conclusions.
I mean really cannot distinguish between assumptions and conclusions; not even (especially not even) when the difference has been pointed-out.
Neither is this basic inability a trivial matter, but is a radical fact of modern discourse. And it leads on to unbounded evil consequences because the devil is inaccurate (to quote Charles Williams - meaning evil does its work by building-upon falseness).
Note: I could not recommend highly enough this essay by C.S. Lewis to anyone with an interest in medieval thought, or indeed the history of ideas. It is simply superb.