But some one will ask, how was it that the Son of God, brought up by His parents with such care, could be left behind from forgetfulness? To which it is answered, that the custom of the children of Israel while assembling at Jerusalem on the feast days, or returning to their homes, was for the women and men to go separately, and the infants or children to go with either parent indiscriminately. And so both Mary and Joseph each thought in turn that the Child Jesus, whom they saw not with them, was returning with the other parent. Hence it follows, But they, supposing him to have been in the company, &c.
Reproduced in full. Translation: Oxford 1841 (Newman), public domain (Public domain).