原: 論語, 里仁/15
子曰：「參乎！吾道一以貫之。」曾子曰：「唯。」子出。門人問曰：「何謂也？」曾子曰：「夫子之道，忠恕而已矣。」

原: 論語, 衛靈公/3
子曰：「賜也，女以予為多學而識之者與？」對曰：「然，非與？」曰：「非也，予一以貫之。」

一以貫之(일이관지), one [/with] through/[threaded] — understanding many through one or a grand principle

From Yi-in (里仁): Gong-ja (孔子) said, “Sam (參, 曾子), my way is through only one thing.” To which, Jeung-ja  (曾子) replied, “Yes, indeed.” After Gong-ja left the room, literati (門人) asked Sam what the master meant. Jeung-ja answered, “My master’s way is only of faithfulness (忠, as being wholehearted) and compassion (恕, as in compassionate understanding of others).”

From Wi-ryeong-gong (衛靈公): Gong-ja asked Sa (賜, 字: 子貢), “Do you see me as someone learned in many things, and be able to recall them well?” Ja-gong (子貢) replied, “Yes, are you not?” Gong-ja answered, “No, I merely see all things through one.”

It’s debatable as to what this penetrating through one thing (一以貫之) may truly mean, but one way to understand this is that, as already mentioned, faithfulness and compassion is a mere way to achieve this, and looking through the greater context it’s only natural to interpret humaneness (仁) as the way (道) through which all things can be reflected on.