Since the time of Aristotle this relation has been called a category. We have already noted the statement which characterizes the age of enlightenment as one in which the causal category, or let us say the distinction between cause and effect, became the dominant issue. Other periods live with their understanding, with their thoughts, in other categories. Though the ancient Greeks knew the distinction between cause and effect, yet it was far from being the dominant point of view in their search after scientific understanding. Instead of regarding, as we do today, everything as effects which were produced by preceding causes, they saw in every process, in every phenomenon, a means which had a purpose. The category of means and purpose dominated the Greeks. Socrates admired the knowledge of nature displayed by Anaxagoras, the stories he could tell of sun, moon, and stars. But as Anaxagoras had omitted to disclose the _reasonable purpose_ of the processes of nature, Socrates did not think much of such a natural science. At that period the means and the purpose were the measure of reason, the handle of the mind, the category of understanding; today causes and effects have taken their places. Between the golden age of Greece and the era of modern science, the socalled night of the Middle Ages, the epoch of superstition, extends. If then you started out on a voyage and first met an old woman, it meant misfortune for you. Wallenstein cast the horoscope before he directed his troops. "Understanding" was gathered from the flight of a bird, the cry of an animal, the constellations of stars, the meeting with an old woman. The category of that period was the _sign_ and its _consequences_. And according to Lazarus, these things were believed by brains which were by no means dull. "I refer to a name which fills us all with veneration: Kepler believed in astrology, in the _category of the sign and its consequence_, together with the thinkers of the thousand years before him and of his own century. Astrology was a science for many centuries, promoted together with astronomy ... and by the same people." The peculiar thing in this statement is the reference to the category of sign and consequence as a _science_. This category has no longer a place in modern science. May not our modern viewpoint, the category in which our present day science thinks, the category of cause and effect, be equally transitory? The ancients have accomplished lasting scientific results in spite of their "purposes." Mediaeval superstition with its "signs," its astrology and alchemy, has likewise bequeathed to us a few valuable scientific products. And, on the other hand, even the greatest partisans of modern science do not deny that it is marred by various adventurous vagaries.