When Moshe was born his mother put him in a basket and placed it among the reeds on the bank of the river, while his sister, Miriam, stood from a distance to see what would transpire. The Torah relates that Pharaoh's daughter went to bathe in the river and she discovered the Jewish child in the basket. Miriam then inquired if she should call a Jewish wet-nurse to feed the child. Rashi tells us that this inquiry came after Pharaoh's daughter had taken Moshe to a number of Egyptian women, but he refused to nurse from any of them since he would ultimately use his mouth to speak with Hashem.
How did Moshe, who was at that time merely three months old, know that there was something wrong with nursing from a non-Jewish woman? Rav Wolbe (Shiurei Chumash) explains that one imbued with kedusha can sense when something isn't appropriate. Moreover, even certain animals have the ability to sense when something is amiss. Chazal relate (Chullin 7a) that Rebbi Pinchas ben Yair's donkey refused to eat from food that had not been tithed! Rebbi Pinchas ben Yair was on such a high level of kedusha that even his animals were affected to the extent that they were able to sense that the food was forbidden to be eaten.
Rav Yeruchom Levovitz zt"l would say that everything that we find regarding kedusha has a parallel in the area of tum'ah. If we have trouble comprehending a concept of kedusha because of our distance from that spiritual level, we might, unfortunately, have an easier time comprehending the concept when it is portrayed regarding tum'ah. Sometimes while walking in the street a person might suddenly glance in a specific direction and just then behold an indecent sight. He had no idea that exactly then he would witness such a scene; however, his psyche sensed the tum'ah and drew him to turn his gaze in that direction.
Lest we think that developing a sense of kedusha is limited to those who lived in the times of Chazal, Rav Wolbe recounted the following story. Shortly before his Rebbi, Rav Yeruchom Levovitz, passed away, his doctor felt that due to the severity of his failing health, he must eat meat which was not salted to remove its blood. This would be halachically forbidden under normal circumstances. When they served Rav Yeruchom his meal, he put the meat in his mouth but immediately spit it out, declaring that it was "non-Jewish meat." His level of kedusha enabled him to sense that the meat was not kosher.
Kedusha and tum'ah aren't merely abstract concepts of Judaism; they are a reality. Rav Wolbe once commented to a group of former talmidim, that his main goal in all his discourses was to convey this very idea that ruchnius is a reality!