Nature abhors a vacuum.
That axiom, attributed to Aristotle, is the principle through which one cleans his carpets and moves liquids to defy gravity. By creating a vacuum in a vacuum cleaner or in a siphon, a force is created that causes dirt to be pulled out of carpets and liquids to fill a tube.
But it seems from Chazal that this rule applies not only to physical matter but even to human nature. The human spirit also abhors a vacuum, and will fill the vacuum either with kedushah or with tumah.
The following Midrash indicates as much:
Whenever Torah wishes to enter and find the chambers of one's heart clear [of sin], it enters and dwells within it, [after which] the yetzer hara cannot take control, nor can [the Torah] be removed.
This is comparable to a human king who, while traveling in the wilderness, comes upon a mansion with spacious rooms. He enters and dwells within it [and cannot be removed]. Conversely is the case for the yetzer hara: If the yetzer hara finds one devoid of Torah, it takes control and cannot be removed.
This "either/or" relationship between Torah and tumah is highlighted also in Rabbeinu Yonah on the Mishnah in Avos that states:
A person who speaks excessively with women brings misfortune upon himself, removes himself from Torah, and ultimately is confined to Gehinnom.
Rabbeinu Yonah comments:
For Torah concepts cannot be placed before one whose mind is focused on women, or socializing with them. One's mind cannot maintain both [Torah and tumah] simultaneously.
This idea can be understood as consistent with the general observation that the vacuum of kedushah is replaced by tumah.
For example:
When a person dies and the soul leaves the body, the kedushah is replaced with tumah. Therefore, the corpse of a Jew generates more tumah than that of a non-Jew (specifically as regards tumas ohel), because a Jew's neshamah is from a higher source.
A woman is tamei after giving birth because she no longer is housing the neshamah of a Jew within her.
Saturday night is known to be a time of tumah because it is when the kedushah of Shabbos departs.
The place in the world that once housed the greatest kedushah, the Holy of Holies in the Beis HaMikdash, is now a house of worship for a barbaric and violent religion.
One's mind is the repository for Torah.
Even before one is born, Hashem fills the mind of a Jew with Torah. This creates the either/or tension. One can fill his mind with Torah, in which case the tumah cannot enter. But if he fails to do so, tumah will be drawn to it as with a vacuum, as it is always drawn to a place that once housed, and that has the capacity, once again, to house the kedushah of Hashem's Torah.