In the famous scene at the end of the parsha, Avraham's servant returns from Padan Aram. He brings home a very special woman, the woman who is to become Yitzchak's life partner.
"Isaac had just come back from the vicinity of Beer Lachai Ro-ee; He lived in the Negev. And Isaac went out to converse in the field towards evening, and looking up, he saw camels approaching. Raising her eyes, Rivka saw Yitzchak. She alighted from the camel and said to the servant, 'Who is that man walking in the field towards us?' And the servant said, 'That is my master." And she took her veil and covered herself."
Chazal in a famous passage suggest that Yitzchak was praying.
"Isaac instituted the afternoon prayer, as it says, and Isaac went out to meditate in the field towards evening, and 'meditation' means prayer, as it says, 'A prayer (tefilla) of the afflicted when he faints and pours out his meditation (sicho) before the Lord.'" (Berachot 26a)
Rashi adopts this reading of the verb "lasuach" - that Yitzchak was praying in the field. However, from a purely textual basis, the words fail to specify that Yitzchak is engaged in prayer. Is the verb "lasuach" necessarily indicative of prayer, or possibly Yitzchak was conversing with another individual?
The Ibn Ezra suggests that Yitzchak
"Went to walk through the plants."
He sees the verb "lasuach" as related to "siach" – a plant! Hence Yitzchak goes out in the cool evening hours for a walk in nature! The Rashbam takes a similar approach:
"He went out to plant trees and to talk to his workers."
Or the Ramban who suggests that we are dealing with an actual conversation:
"He conversed with his associates and friends."
It would appear that the Ibn Ezra and the Ramban have a good textual basis for their translations. They seem more literal somehow. We wonder why the Rashi chooses the unusual option of prayer. How did Rashi decide that prayer was the appropriate meaning here?
Now some might suggest that Rashi frequently resorts to Midrash. However I believe that in this cirumstance , there is a more solid foundation for Rashi's choice. Nechama Leibowitz notes that the word "siach" as in plant, or tree, never appears in the grammatical form of a verb. It has only the Noun form. However, if you open a Concordance (or use the Bar Ilan disk!) and look up the verb "siach" as a verb. you will see that the verb as it appears in Tanach comes up almost exclusively (20 times!) in the context of self contemplation (in a religious context) or talking about God. The form of the verb is never used to describe a mundane conversation between people. And so, from a "Bekiut" knowledge of Tanach, Rashi deduced that Yitzchak must be meditating or thinking about God.
Possibly the Sephorno's reading is the most suitable:
"Yitzhak went to meditate in the field: He went off the path in order to 'pour out his conversation' to God. (He choseto pray specifically in) the field so that he would not be distracted by passers-by… and even before he prayed, he was answered (by Rivka's arrival.)"
Yitzchak knew that Avraham's servant was on a mission to find him a suitable wife. He was davening for a good "shidduch." And sure enough! He had barely started to pray and his prayers were answered!
 A view shared by theTHE Rasag and Rabbeinu Chananel
 See Radak
 In her book on Rashi's Commentary – a University course for the Open University (written with Moshe Ahrend)