  An important application of this idea of economy occurs when two theories, formerly separate, come in contact. For Mach this is a central concern: he is driven to unify psychology and physics. At issue here is the economical requirement of needing a single orientating perspective: “But anyone who has in mind the gathering up of the sciences into a single whole, has to look for a conception to which he can hold in every department of science.” [AS: 312] When theories come in contact with each other, adaptation of one to the other must take place: “Epistemological criticism [is not a problem for the physicist]. But when it is a question of bringing into connection two adjacent departments, each of which has been developed in its special way, the connection cannot be effected by means of the limited conceptions of a narrow special department. By means of more general considerations, conceptions have to be created which shall be adequate for the wider domain.” [AS: 313] It is uneconomical to have one theory for physics and another for psychology; the mind demands that the two be brought together. Attempts at such mutual adaptation have so far been failures and Mach hopes that his ideas can facilitate this: