Isaac is the second of the ushpizin and is associated with the Sefirah of Gevurah (‘judgment’) among the seven middot. Gevurah carries with it the ideas of measurement, restriction and concentration. Where Chesed is limitless, Gevurah sets a boundary. This idea is a requirement in order for anything to take ‘form’ and exist,
Isaac is a peculiar character in the Torah in terms of how little is said about him. What we do know does exemplify several aspects of Gevurah in that his life was one of ‘precise’ actions that reflect this:
• First to be circumcised at the age of eight days.
• Accepted the wife his father had arranged for him.
• Did not protest Jacob’s stealing of the birthright or defend Esau. He sticks to the ‘letter of the law.’
• Did not leave the land of Israel.
• The Akedah, the binding of Isaac by Abraham, which of course depicts the idea of restriction.
Gevurah includes the aspect of ‘concentration of force,’ which can be applied to pushing aside the superfluous to extract deeper spiritual riches. We see this where Isaac went back to his father’s wells and dug them deeper.
Another term used in place of Gevurah is ‘Pachad,’ which is ‘fear.’ This comes from ‘Pachad Yitzach’ in Genesis 31:42, with Jacob speaking to his father-in-law, Laban, where he says:
“Unless the G-d of my father, the G-d of Abraham and the Fear of Isaac, had been with me, surely now you would have sent me away empty-handed…”
The ‘Fear of Isaac’ in this verse is presented as a name of G-d. Jacob is calling out the attribute of ‘fear’ relating to that which enabled him to be as cunning as Laban. He was ‘serpent-like,’ which is another aspect of Gevurah and ‘left side’ of the Tree of Life.
Whereas Abraham was about proactive chesed without concern for limits, Isaac reflects the traits of being contemplative and restraint. This does not mean that chesed and gevurah are necessarily opposed to each other. They work together just as Abraham and Isaac did.
http://www.13petals.org/topical-studies/ushpizin-movie-web/ushpizin-themes/ushpizin/
