Biblically, the Sabbath is a day on which we must desist from performing certain acts. The prophets and the sages, however, instituted that on the Sabbath we must also refrain from speaking of these acts. Citing this verse, the sages state: "Just as God's Sabbath was a respite from speech [since God created the world with speech], so should we refrain from mundane speech on the Sabbath." They then relate the story of an exceptionally pious man who, one Sabbath, saw a breach in his fence and decided to fix it after the Sabbath. When he later realized that he had been thinking about work on the Sabbath, he vowed never to fix the breach. So, as a measure of piety, we should not even think of non-Sabbath acts on the Sabbath.

The reason why we are not biblically obligated to imitate God in speech and thought is because God's thought and speech create, while ours do not. Hence, from a biblical perspective, we are only prohibited from desecrating the Sabbath through our deeds, which can be compared to God's creation of the world.

Speech, which can affect the physical world indirectly (by influencing or directing how others act, for example), is similar to creation and is therefore forbidden rabbinically. Rabbinic enactments take us beyond the strict letter of the law, reflecting the deep, innate Jewish desire to cleave to God's ways.

Thought, in contrast to both action and speech, is an entirely self-oriented faculty. It bears no resemblance to the outwardly oriented act of creation, and is therefore permitted. A pious person, however, who seeks to merge his own identity with God, will cease from even mundane thought on the Sabbath, in imitation of his Creator.

Adapted from Likutei Sichot, vol. 11, pp. 80 ff.