Yesod is the sephira near but not quite at the bottom of the tree of life, a kind of gathering point for all the energies present in the other sephirot before they trickle down to reality, which is next week's subject. (Oddly enough my work involves a form of flux analysis in metabolic networks). It's identified with the "procreative organ" as the books so delicately put it. What does righteousness have to do with it? Well, the very first commandment was to be fruitful and multiply. Jews are obligated to make more Jews. The tzadik is the foundation of the world.
That's a very strange thought to my ususal materialist self. If righteousness and persons are anything at all, they aren't the foundation of the world but a late, epiphenomenal, accidental sort of thing. But maybe not. Maybe the universe was inevitably directed towards making entities that could perceive it (via anthropic selection if nothing else), and maybe such entities had to have a moral sense baked into their foundations, maybe it's an inevitable and necessary a part of intelligence. I can almost see that.
Also odd to me is the confluence of (pro)creation and righteousness. I tend to view creation as an amoral, wild sort of activity, whether it is influenced by the divine or not, it proceeds by its own rules and not some external law. But again, maybe not. Again, going back to the earliest parts of the Torah, the message is that God not only created the world but pronounced it good. And when I create something, I generally have some idea of goodness in mind that guides me. So maybe it's not so strange, maybe my ideas about creativity have been infected by a pernicious romanticism or something.
A fleeting idea of a sort of metaphysical Darwinian process: that which exists is good, what is good exists, and what exists is that which is capable of propagating itself, of procreating, of having its form persist and replicate across space and time. A tzadik is one who combines the moral, physical, spiritual, biological, and I-don't-know-what-other forms of this process into one handy human-shaped container.
Alright, that's enough of the acid flashbacks for now, got to walk the dog and take out the garbage here on planet Earth.
[and, let me just note, that while "what exists is good" may have some sort of truth in a visionary sense, that kind of thought doesn't survive for an instant once critical thought from the merely human perspective is applied to it. From that vantage, all sorts of existing things are manifestly not-so-good, from the Holocaust to polio to global warming to mundane everyday problems (we just had to have all of the heating ducts in our house replaced because they had originally been installed by incompetents and have been mostly eaten into by the local wildlife...) to all the individual tragedies of life (another suicide on the railroad tracks I ride to work on last night). The pollyanish, best-of-all-possible-worlds kind of attitude that sometimes accompanies religion is one of the major turnoffs/obstacles for me. But at least I can catch a glimpse of where it's coming from.]
Speaking of that: last week we touched on a line from the Torah that resonates with the above. Abraham tries to talk God out of destroying Sodom and Gommorrah and says "shall not the judge of all the Earth do right?" (Genesis 18:25). I was somewhat embarrassed that I recognized this from Alan Moore's Watchmen comic, but no matter. It's rather startling, in that we see a human teaching morality to God, chiding him even. There's a major part of the Jewish character rooted in that, a more familiar one than this mystical stuff, which seems linked with an unquestioning acceptance. Also contrast with Christian theology which has a whole branch dedicated to explaining away God's shitty behavior.
[[Update: Stumbled on this passage just after writing about my personal duct problems above:
Yesod means "foundation", and the sephira represents the hidden infrastructure whereby the emanations from the remainder of the Tree are transmitted to the sephira Malkhut. Just as a large building has its air-conditioning ducts, service tunnels, conduits, electrical wiring, hot and cold water pipes, attic spaces, lift shafts, winding rooms, storage tanks, and a telephone exchange, so does the Creation; the external, visible world of phenomenal reality rests (metaphorically speaking) upon a hidden foundation of occult machinery.