D O C U M E N T 4 2 1 M A R C H 1 9 2 9 3 8 5 immediately topical research themes upon which the new institute can develop its activities to particular advantage. Theoretical physics cannot succeed in institutions where no experimental work is possible. Every theoretician arrives in his work at a point where he can go no fur- ther as long as this or that consequence of his thought processes cannot be experi- mentally tested and determinations made that select from the manifold of purely intellectual possibilities the single one that is most relevant. The field, with its cur- rent rapid progress, cannot wait until somewhere in the world a suitable figure is found who is willing to undertake such a test. For although in the course of time, someone, somewhere always takes on this task, still the uncertainty and delay is a disadvantage to all of those fields that can profit from successful progress in theory. While the experimental physicist sees experiments as tools, they are signposts for the theoreticians, which show the proper direction at a crossroads. Times in which theoretical physicists need not make use of experiments can be characterized as times in which trial-and-error has gotten ahead of understanding, and empiri- cism has gotten ahead of theory. In our time, the opposite is the case. For this reason, the proposed institute can- not long be limited to a small archive building, which we suggested at the time of its founding more from considerations of the limited funds available than from a different interpretation of its needs. On the contrary, it will be necessary to con- struct a building in which experiments can also be carried out. A quick estimate shows that a structural area of ca. 600 m² with four functional floors will be required. The available operating funds for such an institute will also have to be considerably increased, to a level that is readily indicated by a compar- ison with the budgets of the other Kaiser Wilhelm institutes in Dahlem. The location at which such an institute might be erected has often been debated within the Kaiser Wilhelm Society. Since the year 1917, the idea of placing a phys- ics institute between the two existing chemical institutes has been floating about these would have the closest scientific relation to a Kaiser Wilhelm Institute for Theoretical Physics, and their directors and members would warmly welcome the construction of a physics institute in their immediate neighborhood. The available land there is favored not only because it represents the geometric center of a scien- tific circle that seeks contact to a breeding area for theoretical physics, but also be- cause it includes a sufficient amount of space for expansion, as the institute can be expected to demand and require, given the development of theoretical physics in the recent past, in the case that the Kaiser Wilhelm Society proves willing to lend its accustomed firm support to this great task of scientific development, which has up to now been insufficiently promoted in relation to the needs of the field in Ger- many. (signed: Einstein, Haber, von Laue, Nernst, Paschen, Planck, E. Warburg)