Philo Judaeus De Posteritate Caini 134

Now of the four virtues, some are always virgins, and some from having been women become changed into virgins, as Sarah did; "For it had ceased to be with her after the ^{1 2 3} manner of women," ⁴ when she began to conceive her happy offspring Isaac. But that which is always a virgin, is that of which Moses says, "And no man whatever knows her." For in truth, it is not permitted to any mortal to pollute incorruptible nature, nor even clearly to comprehend what it is. If indeed he were able by any means to become acquainted with it, he would not cease to hate and regret it;