A realization of the top-down use of knowledge in the process of extraction of simple man-made objects from aerial photographs is presented. The finding of objects is performed by a reasoning rule-based program written in prolog. The program is purely symbolic and has no access to digital data, yet it produces the needed objects by controlling three other modules of the system through a purely symbolic interface. The program was run as a road finder on aerial images and the experiment is described in detail. We demonstrate the simple "programmability architecture" of the program by presenting examples of simple additions that make it find different kinds of simple objects. Our paradigm makes it possible to understand much better, via the reasoning mechanism, the process of object extraction.