De Opificio Mundi

Again, the dangerous diseases of the body, especially when lasting fevers, arising from the distemperature of the powers within us, attack us, are usually decided about the seventh day. For that day determines the contest for life, allotting safety to some men, and death to others.

And the power of this number does not exist only in the instances already mentioned, but it also pervades the most excellent of the sciences, the knowledge of grammar and music. For the lyre with seven strings, bearing a proportion to the assemblage of the seven planets, perfects its admirable harmonies, being almost the chief of all instruments which are conversant about music. And of the elements of grammar, those which are properly called vowels are, correctly speaking, seven in number, since they can be sounded by themselves, and when they are combined with other letters, they make complete sounds; for

On which account it appears to me that they also originally gave letters their names, and acting as became wise men, did give the name to the number seven from the respect[*](The word used is σεβασμὸς, as if ἐβδομὰς were derived from that; and the Romans formed septem from ἑπτὰ, by the addition of s. ) they had for it, and from regard to the dignity inherent in it. But the Romans, adding the letter S, which had been omitted by the Greeks, show still more conspicuously the correct etymological meaning of the word, calling it septem, as derived from σεμνὸς, venerable, as has been said before, and from σεβασμὸς, veneration.

These things, and more still are said in a philosophical spirit about the number seven, on account of which it has received the highest honours, in the highest nature. And it is honoured by those of the highest reputation among both Greeks and barbarians, who devote themselves to mathematical sciences. It was also greatly honoured by Moses, a man much attached to excellence of all sorts, who described its beauty on the most holy pillars of the law, and wrote it in the hearts of all those who were subject to him, commanding them at the end of each period of six days to keep the seventh holy; abstaining from all other works which are done in the seeking after and providing the means of life, devoting that day to the single object of philosophizing with a view to the improvement of their morals, and the examination of their consciences: for conscience being seated in the soul as a judge, is not afraid to reprove men, sometimes employing pretty vehement threats; at other times by milder admonitions, using threats in regard to matters where men appear to be disobedient, of deliberate purpose, and admonitions when their offences seem involuntary, through want of foresight, in order to prevent their hereafter offending in a similar manner.