The very idea of science implies a certain degree of separation of the scientist from the rest of the world. The "outer" world (nature) is to be represented in the material forms of a different kind, which, in addition to being regular things, also serve to represent the scientist in nature. This mutual reflection is implicitly dependent on the stage and direction of cultural development, comprising both material production and spirituality (the historical forms of subjectivity). In its full development, the object area of science is to coincide with the whole cultural domain, which is eventually to embrace and creatively rearrange all the world. This objective aspect may provoke an illusion of the (at least principal) possibility of some ultimate science explaining anything at all.
However, this can never be anything but illusion. The very separation of science from its object, as well as the difference of the material of the scientific image of the object from the matter of the object, suggest the thought of an essentially partial representation of the world in science, leaving enough room for other modes of reflection. The closest relatives, also representing nature in conventionally natural forms, are the arts and philosophy; however, there are other levels of reflection that do not oppose the image of a thing to the thing itself (syncretic and synthetic creativity). Just like science, every other level of this hierarchy is to asymptotically encompass the whole world; this does not remove their qualitative difference (that is, the distinction of their products). A keener sight might discover that the overall growth of the domain of reflection is intimately associated with mutual reflection of the different levels, which will thus transgress their original limits and get saturated with newly adopted elements, until there is virtually no difference, and this particular categorial structure is no longer adequate. That is, fully developed science is no longer science, and doing anything scientific way means self-restriction, selection of a definite level of treatment, a specific scale.
Science in general is a limited reflection of the world; this results in unfolding both a hierarchy of special sciences and an inner hierarchy of science, distinguishing, in particular, the empirical, theoretical and methodological levels. Any individual science is obviously bounded by the limits of its object area; every component of science cannot exist without the other components, complementing its inherent insufficiency. However general, none of these specialized formations can pretend to a comprehensive representation of the whole. This principle is entirely applicable to the evaluation of the power and limitations of scientific theory.
There are no all-unifying theories, and the very idea of a comprehensive theory is logically inconsistent. Moreover, it is the extremely narrow character of a higher-level abstraction that makes it so robust and practical: fundamental theory is not applicable to a thing, but this opens wide vistas for adaptation. The traditional idea of generality treats special theories as minor branches of a more general theory, which, in its turn, is derived from some universally valid scheme. This is a mirror-reflected picture of real development, which grows abstractions from numerous typical examples and efficient modes of action. That is, cognition is to cut the branches of a living tree, to get to the bare trunk, and this kills the tree as it were, making it just the raw material for woodworking. Still, this experience brings us to the idea of the presence of the trunk in the many trees we do not care to cut down. This theory may seem most fundamental, as we discover a trunk every time we come closer to a tree; however it is to eventually fail in some new areas of experience, with the twofold effect on our trunk science, either expanding the notion of a tree, or admitting the existence of the entities other than trees. Both solutions represent the aspects of the same: a theory of everything can no longer be trusted as such.
Scientists may strive for a uniform explanation of very different experiences, they may construct absolutely general theories containing all the other theories as special cases; however, some day, yet another experience is bound to come that won't fit in the seemingly comprehensive theorization. The world is qualitatively infinite, and no theory can describe any of the world's turns. Time is the other side of this qualitative infinity: things change, and this devalues the universality of any science, and hence that of any single sublevel.
With that in view, what is the use of modern integrative initiatives, like the unified field theory? Yes, one can show that all the existing field theories can be derived as special cases from a single theory with enough spatial dimensions. What of that? In principle, this could be predicted from the very beginning, since all the field theories are based on the same logical scheme, which makes them a priori combinable in a single theory of the same kind. Obviously, the particular ways to construct such a unified theory may differ, and one could put forth the program of the search for observable effects favoring one of the possible solutions (or all of them). Suppose we can overcome the technical difficulties and complete this work. Does it give us a clue to understanding anything except a narrow class of physical processes? Even admitting that every material thing consists of particles and fields, we cannot reduce the whole world to these partial manifestations. In particular, any collective motion is qualitatively different from the motion of the constituent bodies, and there is no way to entirely explain higher-level effects on the basis of their lower-level mechanisms.
Any theory reflects our current experience in our close environment (albeit extended to the cosmological distances and energies). Being essentially anthropomorphic, physical theories just cannot be extrapolated to the whole world in a straightforward manner, like many scientists do, to impress ignorant sponsors and journalists, for cheap popularity, thus collecting money for serious research. All the talk about the Big Bang, the expanding/collapsing Universe, dark matter etc. is nothing but a kind of pun, a mental game without too much pretense, just to imagine what happens if... Such over-extrapolations may be useful within science to clarify the logic of a theory and outline the limits of its applicability. In this function, they do not refer to any physical reality but the reality of the human thought. Presenting such prototypes of some future theory as absolute truth and the highest achievement of science is always an ideologically motivated act, stretching a formal scheme to support a political claim. This ideological load has nothing to do with science; still, some (former) scientists can be dragged into the fraud by lies and psychological manipulation.
In the economy based on the division of labor, science is always incorporated in many individual sciences never reducible to each other. You may call one of these science an all-embracing super-science; this does not change the very fact of its singularity, its being one on the many. The hierarchy of generality can always be folded and unfolded another way, so that no theory can be universal in an absolute sense; this essential relativity is due to the very separation of any science from its object area. The immediate corollary is that any individual theory must deal with something particular, and never speak about the whole world, taken in all possible respects. With too wide generalizations, we inevitably drift from the domain of science into the realm of philosophy (which is not science).
Since any cultural distinctions in science (both informal and institutionalized) originate from the current structure of human activity, and hence are application-bound, any science is to keep within its cultural niche, on a certain level of hierarchy, developing models of a limited relevance to involve a very specific range of phenomena. As human activities evolve, sciences mutate into other sciences, treating other (but as specific) phenomena. However general, an individual science is restricted to only one of the infinity of the possible relations of the humanity to the world; this concentration on the product gives science its influence and strength, making it truly practical.
The impossibility of all-unifying science does not mean that a result obtained within one particular science cannot be used in another. However, such a scheme transfer is never accomplished through mere extrapolation, but rather employs the mechanism of activity exchange. People learn from each other doing different things in a similar manner, and one science can borrow certain tricks from another (or even from any non-science), adapting them to a different context. With all the superficial similarity, the sense of the same method is bound to change from one science to another. A formal apparatus borrowed from elsewhere needs to be reinterpreted and adapted to the description of the host object area. Quite often, this implies drastic modifications; there is no "exact" reproduction of a formalism, since the same construct is to refer to a different class of things, with its specific constraints.
As yet another aspect of that mutability, note that the very idea of comprehensiveness is beyond science and scientific everything is different from the whole of any other vision of the world, as well as the individual science do not share the same notion of completeness. In the developing world, science develops as well, never reaching any limits: you can talk about everything, but you will never tell everything about it.
Science is to produce approximate models of the world, and no such model can pretend to describe the whole universe, albeit in an artificially isolated domain. In certain respects, the diversity of scientific theories can be ordered by a kind of generality, but, as in any hierarchy, each level retains its specificity and cannot be reduced to any other level. Moreover, in a different cultural context, this hierarchical structure can be unfolded in a quite different manner, with formerly "special" sciences becoming more general than the former unified descriptions. Apparently, such hierarchical conversion means scientific revolution; but who can swear that the present picture of world is already complete and shaped forever?