One of the books that I have read over Christmas has been the new biography of Louis Kahn by Wendy Lesser (You Say to Brick), which dishes the dirt on his multiple affairs with young architects in his office.   But much more interesting than the affairs – to me at least – was the fact that from the age of eleven he was able to take drawing classes at the Public Industrial Art School in Philadelphia, where he was taught by a man named J. Liberty Tadd, who had been a pupil of Thomas Eakins at the Pennsylvania Academy of Fine Arts.   Tadd had developed what he called a system of ‘natural education’ which developed skills of visualisation through the process of drawing and, also, making, skills which Kahn evidently retained not just in his ablity to draw and sketch buildings when he was travelling round Italy, but also in his ability to imagine and conceptualise buildings in three dimensions.   He was a Beaux Arts architect as well as a modernist.