The Messilat Yesharim is a guide to help people grow spiritually. There are a number of steps that help achieve this goal.
One of these steps is called נקיות, trying to achieve cleanliness. This involves carefully observing all of the Mitzvot of the Torah. There are three areas in particular that require greater attention.
The Messilat Yesharim lists these three in the order of which are the most difficult to overcome. The Mitzvot involving theft are many more than we realize. We need to be “clean” in all aspects of money matters. This could be even harder than overcoming matters connected with illicit relations. This is because, areas related to money are much more subtle, and not so obvious.
The third area is related to matters of Kashrut. There is also a tendency here to be lax, and not take this as seriously as we should. The Ramchal, author of Messilat Yesharim, points out that we don’t realize the harm that we cause ourselves when we ingest foods that are not kosher. Such impure food has the effect of מטמטם את הלב והמוח, that it causes great confusion to one’s heart and brain.
He goes as far as to say that one should look at food where there is even the slightest doubt, as to its permissibility, just like poison. One would never consciously swallow anything that contains even an ounce of poison. The potential danger is enough to prevent us from taking chances.
The same is true of forbidden foods. The damage that it does is so great, that if we only recognized it, we would take the greatest care to watch what we eat.
These are the steps that we must take in order to grow in spirituality. Recognizing our temptations, and overcoming them, is the way to achieve this goal.