Sufism

Four doors must be known to know oneself: knowledge of self, of God, of this world, and of the next

Ghazali builds his entire practical guide to happiness on the claim that self-knowledge is the foundation without which knowledge of God, the world, and the afterlife cannot be properly organized.

Book 1 of the Kimiya-yi Sa'adat lays out four categories of knowledge Ghazali treats as the necessary architecture for a coherent spiritual life: knowledge of oneself, of God, of this world, and of the world to come. He places self-knowledge first not as one item among four equals but as the doorway to the rest, invoking the widely cited principle that whoever knows themselves knows their Lord — understanding one's own nature, its capacities, its weaknesses, its true source, is the template by which the higher forms of knowledge become intelligible rather than abstract. Without this foundation, a person's relationship to the world and the hereafter tends to be organized around the wrong priorities, comfort or fear, rather than genuine understanding of what each is actually for. This four-door structure functions as the book's table of contents in substance, giving readers a diagnostic order to work through rather than a loose collection of spiritual advice.

Kimiya-yi Sa'adat (The Alchemy of Happiness)