Natural illumination. Of course, this all depends where you live and what your window orientation is. At my latitude, I don't have to worry about not getting enough solar penetration into a room because at mid-winter, the sunlight penetrates almost 5 metres.
Louis Kahn (from "Licht und Raum- light and space", Urs Büttiker) The chronological listing of projects traces Kahn's development process in light control and modulation. Projects beginning with student work from 1924 and ending with the Berkeley library from 1971-1974 reveal an extraordinary broad spectrum of solutions for the design of light and space. Kahn's struggle to find the coherent answer for each architectural challenge remains the basic principle characterizing his complete work.