"The entire commandment which I command you this day..." (Deut 8:1)

Precisely which commandment is Moses talking about? Besides, had he not already spoken about the need to perform all the commandments in verse 12 of the last chapter? What reason was there to repeat the same thing again at this point?

Moses was a clever psychologist...

Moses was a clever psychologist, familiar with the workings of the human mind and psyche. He realized that there is a built-in tendency in us to become progressively more lax in our observance, especially if we have performed a number of commandments meticulously and have chosen to perform one or two commandments with special attention to all its details. Once one has done this, he is apt to look upon oneself as a pretty good Jew and thereafter tend to be less punctilious with the performance of other commandments.

People who are preoccupied with Torah study have a tendency to pat themselves on the back and to disregard some of the commandments which they view as being of minor significance. Such neglect is bound to result in such people suffering various kinds of afflictions; they may experience that others do not show them the respect which they feel is their due, etc. Moses therefore admonishes each and every one of us not to forget to perform the whole (range) of G‑d’s commandments. The only effective way Moses found to bring this point across to us was by describing all of the 613 commandments as one single Commandment, "all the mitzvah."

He resorted to a way of illustrating his point which is incontrovertible, based on the Zohar. (I, 170) According to the Zohar, man has 248 bones and 365 sinews. G‑d commanded us 365 negative commandments and 248 positive commandments, one each to correspond to each of these parts of our body. If one suffers a pain or hurt in one of his bones or sinews he should examine which one of the commandments he had neglected so that the pain in question might be due to such neglect. A man will not be satisfied if in response to his cries of pain caused by the afflicted bone he is told by his doctor that seeing that 247 of his bones do not hurt him he should not really be complaining but count his blessings.

We cannot merely pride ourselves on the commandments we do observe...

We must relate to each one of the 613 commandments in a similar fashion. We cannot merely pride ourselves on the commandments we do observe but must be keenly aware of the damaging effect on our spiritual health of any commandment which we fail to observe when we have the opportunity. All of these considerations are included in Moses’ reference to the need for us to observe the whole commandment, both the negative part and the positive part. The reason for this is "so that you will live and multiply and come and inherit the land." Inheritance of the land of Israel is conditional on commandment fulfillment, which in turn is the guarantee of our spiritual and physical wellbeing.

You may query that in the parable we presented the example does not match the subject matter of the Torah. In the parable we described how the hurt experienced by a single organ upsets the entire body’s balance, something that is not the case when applied to mitzvah performance. This is why Moses had to describe the commandments in the singular. He wanted to make it easy for us to appreciate that when the 613 commandments are viewed as a single unit that the non-observance of even a single one of them can destroy the entire balance of the fabric and throw everything off-kilter.

[Selected with permission from the five-volume English edition of "Ohr HaChaim: the Torah Commentary of Rabbi Chaim Ben Attar" by Eliyahu Munk.]