Mat 15:17 Do you not yet understand, that whatever enters into the mouth goes into the belly, and is cast out into the toilet?

This verse is a gem because it clarifies Christ's theme about physical reality tying together his most important symbols for the physical: food, movement, the belly, and eventual decay. The original Greek is all about the physical, but that is lost in the English. Christ's primary teaching about the physical is that it is neither good nor bad, but that it is temporary, a realm of change and transition. Here, what is good, food, becomes something useless, waste. What is originally sought out is rejected. Remember, Christ doesn't use words like "bad" or "evil." The words that are translated into English that way are really words that mean "useless" and "worn out." Here, he describes the whole cycle of the physical when disconnected from spirit, mind, and relationships.

A better translation: "Don't you see? Whatever goes into the mouth moves forward into the belly and is sent out into the toilet.

A symbolic version: Isn't physical reality obvious? What is good physically just goes into our bodies where it is changed and moves forward to something worthless to our bodies.

"Enters into" and "goes into" have nearly the same meaning in English, but the Greek is clearly two very different ideas. "Enters into" is eisporeuomai, which specifically means "goes into," while "goes into" is choreo, which means "to leave room," "to make way," "to advance" or "to proceed."

"Understand" is a special word that Matthew chose precisely to a discussion of the physical. The Greek word, noeô, means specifically "to perceive with the eyes." This is important because "eyes" are the sense that Christ dedicates to the physical. Hence, the "blind guides" that begin this verse. Many other words here would have put "understanding" in the mental realm of ideas, but not this word.