SEMAINE D'ÉTUDE SUR LE ROLE DE L’ANALYSE ECONOMETRIOUE ETC. 
105 
trary, it is my view that this is a process which must be regarded 
with the greatest suspicion. Past experience with theories in physics 
shows that what was fashionable at one time or in a certain period 
was subsequently completely invalidated. We must therefore be very 
critical about the choices made by current opinion, and personally 
I would abstain from taking up any position on the question of 
whether the choices which are presently those of the majority are or 
are not useful or fruitful. TI only think that it is necessarv simply 
to be very careful. 
A fourth point is that there are three essential stages in the con- 
struction of a model. The first, the working out of the basic assup- 
tions follows a process of successive approximations similar to that 
mentioned by Professor STONE. The second stage is purely logical, 
a deductive stage in which essentially mathematical techniques are 
applied. Finally, the third and probably the most important stage 
is the confrontation of the theory and the facts. Further, I believe 
that these are Professor STONE’s ideas, but it seems to me that his 
conception is more neutral, and it appeals to me personally more than 
some of the points of view which have been expressed. I do not 
think that we ought to have a priori or normative ideas about what 
we are going to do. It is first necessary to understand well and to 
describe properly, and the basic aim of those who construct 
models ought to be above all to give as complete information as 
possible, and I would add, as neutral as possible. It is also desirable 
that there exist decisional models in parallel with this, but these are 
completely separate fields. There is the description of facts, their 
explanation, and action, which implies normative choices, but each 
of these stages must be kept separated carefully. Thank vou. 
SCHNEIDER 
What does it mean: « Models must be neutia 
; Stone - pag. 103