The article presents an exchange of letters between Jakob Marschak and Trygve Haavelmo in May-July 1943. Marschak had from the beginning of 1943 become the research director of Cowles Commission at the University of Chicago. Trygve Haavelmo, who at the time worked for the Norwegian Shipping and Trade Mission in New York, had just published the article on the statistical implications of simultaneous equations, which would become his most quoted work. The content and the implications of the article was at the centre of the letter exchange. The introduction provides some background for the exchange.