"One day in 1936, he came to me for advice on further graduate studies. This was our first meeting. I looked up to observe a slight, short young man, with a serious look, who answered my questions with unusual precision. I was immediately impressed with the keenness and quickness of his mind, and I suggested that he enroll at Caltech for advanced study ... Tsien agreed. He worked with me on many mathematical problems. I found him to be quite imaginative, with a mathematical aptitude that he combined successfully with a great ability to visualize accurately the physical picture of natural phenomena. Even as a young student, he helped clear up some of my own ideas on several difficult topics. These are gifts which I had not often encountered and Tsien and I became close colleagues."