The first law of Our Lord will immediately enlighten, for what sort of a crime adultery is to be taken, which as we have stated as a crime is the same thing as fornication. Having forbidden superstitious worship of other gods and making idols of these gods, after behest of observance of the sabbath, after filial piety due to parents next to God enjoined, he had consequently no other commandment to give in order to strengthen and advise on such essential matters than, "Thou shalt not commit adultery." After the chastity and inviolability of spirit followed the purity of flesh and even this he advised on, forbidding its opposite, adultery. And now you can imagine how severe this sin is, which is placed with and immediately after idolatry. Nothing that follows is far away from the first. In fact - nothing is so near to the first as the second, and born from the first in some way is a new first thing. That is why adultery, closely connected with idolatry - by the name of adultery and fornication idolatry has often been punished in Our Lord's own people - is joined with it, with respect to fate as well as to order; and in the same way they will join in damnation as well as in rank, and what is more - when he said, "Thou shalt not commit adultery," he continued, "Thou shalt not kill." At any rate he laid greater stress on adultery, which he mentioned before homicide and which therefore is on the first page of the most holy law among the first lines of this message of Heaven marked with the sign of the utmost cordinal sin. Do you now understand its extension from their place, its quality in their mutual order, its quilt by their neighbourhood? There is a graduation list even for evil: it can be placed on the highest rank or alternatively among the most wicked. [De Pudicitia 5]
The understanding here is even more interesting as it reflects Philo's arrangement of the Ten Commandments:
This, then, may be enough to say about the divine voice. But a person may very reasonably raise the question on what account it happened, when there were so vast a number of myriads of men collected into one place that Moses chose to deliver each of the ten commandments in such a form as if they had been addressed not to many persons but to one, saying:--
Thou shalt not commit adultery.
Thou shalt not steal.
Thou shalt not Kill. [Ex 20:13]
And giving the other commandments in the same form
Having then now philosophized in this manner about the honour to be paid to parents, he closes the one and more divine table of the first five commandments. And being about to promulgate the second which contains the prohibitions of those offences which are committed against men, he begins with adultery, looking upon this as the greatest of all violations of the law ... Since, then, illicit cohabitation produces such great calamities, adultery is very naturally a detestable thing hated by God, and has been set down as the first of all transgressions.
The second commandment of this second table is to do no murder ...
The third commandment of the second table of five is not to steal ...
And after he has forbidden stealing he proceeds in regular order to prohibit bearing false witness, knowing that those who bear false witness are liable to many great accusations, and in short to every kind of terrible charge ...
Last of all, the divine legislator prohibits covetousness, knowing that desire is a thing fond of revolution and of plotting against others; for all the passions of the soul are formidable, exciting and agitating it contrary to nature, and not permitting it to remain in a healthy state, but of all such passions the worst is desire
Why would Philo of Alexandria's unusual ordering of the Ten Commandments have influenced Tertullian of Carthage?
“Finally, from so little sleeping and so much reading, his brain dried up and he went completely out of his mind.”
― Miguel de Cervantes Saavedra, Don Quixote