There are several persistent historical and logical factors to support unity of the sciences.
1. All sciences deal with the same universe.
2. The different phenomena encountered in the universe are interrelated and interdependent. It is assumed, usually implicitly, that the universe itself is somehow unified. Any seeming disunities are therefore assumed to be due to limitations in description and understanding and not to the fundamental reality. Assigning certain phenomena to different disciplines therefore creates artificial, or unnatural, boundaries.
3. Another assumption implicit in the concept of the integration of science is that science (as distinct from nature) is coming to be regarded as unified in substance and content. A finite set of logically related laws and theories should eventually emerge to explain all natural phenomena. New theories are characterized by their applicability to more phenomena than the theories they replace. (Physics is widely regarded as the fundamental science on which all other sciences must ultimately be based.)
4. There are certain key concepts that permeate all science disciplines. These concepts (e.g. energy, equilibrium, system) are relatively few in number and may therefore be regarded as constituting the essence of a unified science.
5. There are fundamental similarities in the manner in which scientists establish new knowledge, regardless of the discipline or field with which they are nominally associated. There is general agreement on a scientific method and such processes as hypothesis formation, validation, use of theories and models, etc. These may be felt to be of greater significance than any particular body of facts, laws and theories that comprise the findings of the separate sciences at any particular time.
The three most familiar conceptions of the unity of science are:
A. Unity as a reduction to a common basis, in which a single descriptive language is devised in which all the terms of all the sciences could be defined or to which they could all be reduced. If a single set of laws emerged, from which all the laws of all the sciences could be derived, or to which they could all be reduced, there would then be a single science.
B. Unity as the construction of an encyclopedia of scientific statements, with all the discrepancies and difficulties which persist.
C. Unity as a synthesis into a total system, in which a model or pattern of scientific theory is elaborated, of which each particular theory is an instance, so that higher level sciences recapitulate, although with more complex elements, the structure of lower level sciences. General systems research has shown that certain principles apply to systems in general, irrespective of the nature of the systems and of the entities concerned. Corresponding conceptions and laws therefore appear independently in different fields of science, causing the remarkable parallelism in their modern development. Thus, concepts such as wholeness and sum, centralization, hierarchical order, equifinality, etc. are found in different fields of natural science, as well as in psychology and sociology.