According to the Torah, “soothsaying and divination” are forbidden (see Vayikra 19:26). The Gemara on today’s daf(=page) limits this in a number of ways. Specifically, the Gemara distinguishes between “divination,” which is forbidden and “signs,” which are permitted.
Rav explains “divination” as follows: An omen which is not after the form pronounced by Eliezer, Avraham’s servant (seeBereishit 24:14), or by Yonatan the son of Shaul (see I Shmu’el 14:9-10), is not considered a divination.
The question raised by the rishonim is how Eliezer and Yonatan were allowed to perform these divinations.
In the case of Eliezer, Avraham’s servant was charged with finding an appropriate wife for Yitzhak. The Ran argues that forbidden divination is only in a case where the signs that are sought out are without reason. In his case, by investigating whether the girl at the well was willing to be helpful to a stranger, he was able to ascertain whether she had the qualities that were appropriate, so this divination was permitted. Tosafot argue that Eliezer did not actually rely on the divination, since he waited to find out whose daughter Rivkah was before giving her the presents (see verse 47; Tosafot views that passage as more accurate than the earlier description of events).
Similar explanations are given for the incident with Yonatan and his servant who determined to go to war based on the response of the Philistine soldiers. The Ran argues that based on their frightened response, Yonatan understood that a small group of brave soldiers would be able to fight the army. Tosafot argue that Yonatan planned to fight in any case; the divination was performed merely to encourage his servant to join him in his plans.
Although the poskim disagree whether it is permissible to make use of indications like these to decide one’s future direction, the Rambam concludes that someone who relies exclusively on his faith in God will be surrounded by God’s grace and protection (Mishneh Torah, Hilkhot Avodat Kokhavim 11:4).