On verso of Portnow's letter. "I should not worry too much if I were you about limited mathematical ability. Many most eminent physicists had no mathematical ability—e.g., Faraday and Rutherford. I think mathematics is more useful to a philosopher than to a 'pioneering chemist', but I expect that when you get away from exams you will find yourself unexpectedly competent at the rather peculiar kind of mathematics that stimulates philosophical imagination."