Yesod (Hebrew יסוד “the foundations”) is a sephirah or knot in the Tree of Kabbalistic Life, located near the base of the Tree, is the sephirah under Hod and Netzach, and above Malkuth (the kingdom). It is seen as a vehicle that allows movement from one thing or condition to another (the power to be connected with GOD and others).
According to the Jewish Kabbalah, Yesod is the foundation on which GOD built the world, and also serves as a transmitter between the sephirot above and the reality below. The light from the upper sephirot collects in Yesod and is directed downward to Malkuth, which is why Yesod is associated with the reproductive genitals. The sexual organs are channels through which the vital semen arrives to fertilize Malkhut, the female vessel that must receive it. Yesod therefore has a great power of fertility, growth and development. These traits are also extended to the figure of the Tzadiq, as he becomes the one who throws seeds into the fertile soil of the mind and psyche. The male Yesod then collects the vital forces of the higher sepirot, and transmits these creative and vital energies into the female Malkuth below. The channels of Yesod, Malkuth are the receivers. In turn, it is through Malkuth that the earth is able to interact with the divinity.
Yesod plays the role of collecting and balancing the different and opposite energies of Hod and Netzach, and also from Tiferet above it, storing and distributing them all over the world. It is compared to the “engine room” of creation.
Since Yesod is responsible for the powers of communication, connection and contact with the external reality within the Soul, unifying the material world of Malkuth with the other Sephiroth, some Christians compare Yesod to the third “person” of the Holy Trinity, the Holy Spirit. The Holy Spirit is defined by some Christians as an aspect of the Trinitarian GOD, believing that the Holy Spirit is the aspect of God that unifies GOD with humanity.
The verse of the Book of Proverbs 10, 25 reads: “tzadiq yesod olam” which means: “the just is the foundation of the world”, so Yesod’s sphere is also related to the figure of Tzadiq, the Just.