By “universal”, I am referring to the object of the problem of universals. I’m looking at how one could use Ayn Rand’s solution to clarify some issues.
preliminaries
“An entity is that which you perceive and which can exist by itself. Characteristics, qualities, attributes, actions, relationships do not exist by themselves.” [Ayn Rand, Introduction to Objectivist Epistemology, workshops, “What is an entity?”]
Whether or not the entities responsible for a specific effect are known, there is something acting in a certain manner.
Any concept we form on the basis of concepts of entities can be applied to those kinds of entities going forward.
Causality is the law of identity applied to action.
Not every “universal” is suitable for scientific investigation:
Invalid concepts arrest further advancement. They directly lead to false theories, such as “phlogiston” or Cartesian “vortices”. Particulars must qualify as units of a valid concept. The concept of the universal must be such that one can objectively determine (i.e. through a process of measurement-inclusion) whether some particular thing qualifies as a member. Leonard Peikoff, David Harriman, and others have touched on this. Explicit knowledge of conceptual context can help reach true conclusions about the nature of the causation, thus providing a solid basis for induction.
If you have valid concepts, what makes one “universal” more suitable for investigation than another?
What is not attributable to entities does not easily offer generalization. The more abstract a concept is, the more levels of abstraction one must traverse in order to interpret factual data about perceptual concretes. The more room there is for error, the more easily one can misinterpret factual data or jump beyond what the evidence affords. Is it better to focus on concepts “closer” to the perceptual level? Is it better to focus on concepts of perceivable attributes, actions, processes, relationships, etc.?
Distinguishing a substance from entities yields definition but few generalizations.
Investigation of a specific kind of attribute, action, process, or relationship provides necessary foundation for explanation. The history from the gas laws to statistical mechanics and atomic theory come to mind. In the 1500s, people knew that liquid water became steam but little else. With the gas laws and the concept of constituents of matter, we could explain boiling as the activity of the constituents overcoming atmospheric pressure.
What is not measurable cannot be quantitatively compared to other instances. This severely limits the range of what can be discovered. It would be better to find the Conceptual Common Denominator, such as what was done for heat, for sound, and for electromagnetism.
What is measurable in more than one dimension requires more work to explain. Some scientists who encountered a pair or triple of attributes attempted to separate them experimentally, as Galileo did with horizontal/vertical motion and Francis Bacon did in his scientific work. Other scientists found themselves unable to separate certain measurements so they looked for quantitative relationship(s) instead, such as Boyle, Amontons, Charles, and Gay-Lussac did when investigating gases.