The shaping or reformulation of the aims of economic policy which are only vaguely felt may be exemplified in the aim of social justice. - Jan Tinbergen

It is my hope that in such a way we may again, as Marx claimed, find scientific arguments in the competition between various systems, but up-to-date scientific arguments rather than obsolete ones. This more fundamental research in economics deserves relatively more attention and resources than the more superficial versions of economic research directed at forecasting or analysing very short-term fluctuations in market prices, on which quite some money is being spent to-day.

First of all I want to remind you of the essential features of models. In my opinion they are: (i) drawing up a list of the variables to be considered; (ii) drawing up a list of the equations or relations the variables have to obey and (iii) testing the validity of the equations, which implies the estimation of their coefficients, if any. As a consequence of especially (iii) we may have to revise (i) and (ii) so as to arrive at a satisfactory degree of realism of the theory embodied in the model. Then, the model may be used for various purposes, that is, for the solution of various problems. The advantages of models are, on one hand, that they force us to present a "complete" theory by which I mean a theory taking into account all relevant phenomena and relations and, on the other hand, the confrontation with observation, that is, reality. Of course these remarks are far from new.

To Ehrenfest I owe a great deal. I studied physics at a time when a number of fascinating persons were there together. Ehrenfest would not instruct as such, as he preferred dialogue. Thanks to him I could participate in discussions with Albert Einstein. Also Kamerling Onnes, Lorentz and Zeeman were present. Being a student in the hands of such teachers, you are very fortunate indeed.