הלכה על קידושין 162:8

And also as they, may their memory be blessed, were like angels, as they had no other occupation - even for a short time - beside Torah and the commandments; and their intention was famous to the whole world like the sun; and they would not feel any evil feeling about anything, due to their great clinging to the Torah and the commandments. But we now should not even breach a small fence in these matters at all, but rather guard all of the ditancinings that they, may their memory be blessed, informed us [about] in particular. And regarding that which they did not mention, each and every one should do according to that which he finds his person ready, as we said. As if he finds about himself that he needs a fence even about the permissible, he should fence himself - as we found (Kiddushin 81b) one of the Sages who said, "Be careful with me because of my daughter," even though it is permitted for a man to isolate himself with his daughter, as we will write with God's will - since this matter is very difficult, and the evil impulse about it is strong.