There are two aspects to this part or the prophecy. The first is that God will raise up a seer. Not a prophet, but a seer. The second is that this seer has a specific calling, and “he shall do none other work, save the work which I shall command him.”
There is no doubt that this seer is Joseph. We not only know Joseph as a prophet, but also as a seer and a revelator. In Mosiah 8:15–16 we learn that “a seer is greater than a prophet” because “a seer is a revelator and a prophet.” We also learn that a seer can use the stones called interpreters and can use them to translate records (Mosiah 8:13). Joseph Smith was functioning as a seer when he dictated this prophecy to his scribe. We can only imagine his feelings as he dictated this prophecy that clearly referred to him and his current task.
Joseph knew the term seer from his environment and childhood. It was a term applied to those who used implements to see what was not naturally seen. Seers used implements that were often called seer stones. Joseph had used a couple of stones prior to receiving the plates. While he did receive the interpreters that Mosiah mentioned, he appears to have translated the majority of the Book of Mormon, as we have it, with a chocolate-colored stone. Photographs of that seer stone are printed in the volume of the Joseph Smith Papers which publishes the printer’s manuscript of the Book of Mormon.
When the prophecy says that Joseph Smith should do no other work, it was a reference to the translation of the Book of Mormon, and not an indication that he should never work, nor ever do any other task. The focus of the prophecy is on Lehi’s seed, and it is the translation of the record of Lehi’s seed that was of greatest importance in the prophecy.