Phi is a letter of the Greek alphabet used in mathematics to represent the Golden Ratio. Mathematically speaking, the Golden Ratio is an impressive equation depicting a beautiful balance of proportion and appears frequently in geometry. It's the equation on which our theories of beauty and symmetry are founded and thus is seen in art, architecture, printing, and design of all types.
The Golden Ratio is often associated with the Golden Square and the Golden Spiral, and is reflected in the Platonic solids. In Nature we see the principle echoed in the spirals of succulents, the stems of plants and the veins of leaves, crystalline forms and even some atomic-level structures. The German psychologist Adolf Zeising, whose research in mathematics and philosophy led to his belief that the Golden Ratio was an expression of Universal Law, is quoted thus:
It is with all of this is mind that I've adopted the lower case form of the letter as my logo, to represent the nature of my approach to asana practice. I have often taught from the perspective of "finding your blueprint". The human form, in each individual incarnation, exhibits a sense of proportion that when applied to our asana practice, offers an ideal foundation and alignment for each of us. It is from this blueprint, this foundation, that I believe our personal practice is most strongly and safely derived.
The symbol itself conjures the image of a sprouting plant, an image close to my heart. I like to think of humans as budding little plants just unfurling our leaves and petals to the ether and sun, ideally while keeping our roots firmly planted in the soil. If you don't look too closely, you can also see a "Y" in the shape of the lower case form — "Y" for Yoga.