In the search for elementary particles, such principles are used as Gell?mann's that ?anything which is possible is compulsory?. This is an example of a teleological principle according to which the scientist tries to realize in science the kind of world that he desires on prior emotional grounds. Mendeleev's classical discovery of the Periodic Law and Table of Elements was thus guided by his mystical values. A mechanistic anti?teleologist such as Jacques Loeb was indeed a crypto?teleologist who wished science to (...) fulfil his radical, socialistic aims. Hoyle's ?perfect cosmological principle? projected conservative values, while such conceptions as Oppenheimer's ?catastrophic collapse?, and Mach's Principle likewise had their teleological bases. Anti?mystical and individualistic values inspired Bridgman's operationism. A ?perfect operational principle? would stipulate that every imaginable state of affairs would be operationally differentiable from others. Teleological principles are essential to the functioning of sciences; in the Theory of Theories, the principle of plenitude may hold: that for every mode of theory, there is some segment of reality for which it provides the best explanation. Naturalistic accounts of basic teleological principles seem inadequate. (shrink)