A number of years ago, I was on a business trip and I had to spend Shabbat Toldot in Overland Park, KS with a very nice family. During the Shabbos table conversation someone asked:
"How could Yitzchak have been so stupid as to like Eisav better than Yaakov?"
The question bothered me and I spent the rest of Shabbos thinking about it. (By the time I came up with one, of course, no one was around to listen.) So here it is:
Even though Yitzchak is spoken about the least of all the Avot, we do know enough about him to glean parts of his personality. Clearly the defining event of his life was the Akeidah, a profoundly spiritual experience. We also know that he spent time meditating outdoors, as when he first saw Rivka he was praying outside. He also was a very accomplished farmer, with G-d granting him unimaginable yields on his crops.
Looking at these examples, it appears that Yitzchak associated spirituality with the outdoors.
Now, look at the initial description of Eisav - Eisav is described as being "a man of the fields," an Ish Sadeh.
A Sadeh is the specific word that described Yitzchak's place of prayer, as well as the place that his father went to considerable trouble to purchase a burial ground for Sarah (the "s'dei Ephron." )
In other words, when given a choice of a son who spends his time outdoors and one who is seemingly a "bookworm" staying in tents, Yitzchak would tend to assume that the "man of the field" is a more likely spiritual heir than Yaakov. Especially since Eisav is the first born.
In other words, Yitzchak could not even imagine a person who could spend time outside and not be a spiritual person. To him, the field was where G-d primarily manifested Himself and it was obvious tht anyone who spent time with nature would see things the same way!
Further proof to this can be seen when Yitzchak is speaking to Yaakov who is pretending to be Eisav:
וַיֹּאמֶר, רְאֵה רֵיחַ בְּנִי, כְּרֵיחַ שָׂדֶה, אֲשֶׁר בֵּרְכוֹ השם
"See, the smell of my son is like the smell of the field that G-d has blessed."
To Yitzchak, the concept of "field" and "G-d" were intertwined. And to a man like this, it seemed clear that Eisav, the man of the field, was the chosen heir.
Perhaps only when he was faced with the juxtaposition of experiencing Yaakov speaking of G-d while smelling of the fields, immediately followed by Eisav's entrance without the reference to G-d, did he realize that his assumption that men of the field had to be spiritual was incorrect. With this realization he reiterated the blessing for Yaakov, later on to add to Yaakov another blessing of "bechira", of being the chosen son to carry on in the ways of Avraham.