1.         The most basic types of observable concepts are those that can be perceived directly, for example colors. Many people extend the category of observables beyond this to include qualities that can be measured simply, for example length, weight, or temperature.  The most restrictive definitions of observable concepts do not include qualities such as weight, because technically one cannot observe an object’s numerical weight itself. Instead, one observes the marking on a scale and then infers the weight of the object from that marking. Even in the strictest definitions, general descriptions of weight (for example, “heavy”) count as observables, because they can be perceived without any use of induction.

Theoretical concepts cannot be observed directly or measured easily. Examples include the mass of an electron or the oscillation of an electromagnetic wave.

Carnap points out that observable concepts blend into theoretical ones as the method of measurement or observation grows more complicated. Since the two terms are somewhat fluid, it is up to individual authors to define exactly how they will use them.1


2. Empirical laws are generalizations of many empirical facts; they express regularities in observable (or easily measurable) phenomena. Theoretical laws describe the behavior of theoretical, unobservable concepts. Theoretical laws help to explain and organize existing empirical laws, and to predict the existence of new empirical laws. However, theoretical laws are not just “extra generalized” empirical laws, because the concepts to which the two types of laws pertain are fundamentally different. Empirical laws are directly supported by observations. Theoretical laws, on the other hand, can only be supported indirectly by the validity of the empirical laws they predict or explain.


3. Without theoretical laws, it would be harder to discover new empirical laws, since theoretical laws often predict otherwise difficult-to-discover empirical laws. The empirical laws that we could discover would also appear more scattered, because theoretical laws provide a unifying structure that underlies and organizes empirical laws. In addition, there would be fewer answers to “why” questions, since empirical laws observe what happens, but only theoretical laws attempt explanations.


4. The most basic way to organize types of existence would be in terms of physical versus abstract existence. Antarctica is tangible, but laws and theories of any type (including Newton’s laws of motion, the theory of evolution, and the law of thermal expansion) are not. However, the products of these laws and theories do exist in a more tangible way. Metal bars really do expand when they are heated, population really do evolve over time, and objects really do accelerate according to F=ma.2 There are other types of intangible existence as well, for example that of universals. While some universals may describe physical characteristics, the universals themselves have no physical existence.3  An object may be blue, but you cannot hold “blueness” in your hand. Subjectivity is one characteristic that distinguishes the existence of universals, especially those that pertain to qualia, from the existence of laws. There is nothing subjective about evolution, but the meaning of all words relating to the senses and to abstract concepts such as beauty and evil relies heavily on the speaker.



1. Perhaps it is useful to define “observable” as a property given to comparison.  Therefore, length is an observable because one can say, “The first plank of wood is clearly longer than the second.”  In this sense, weight would be an observable.


2. Modern physics solves this, to an extent, by reifying forces into fields and particles which do exist (or are claimed to exist).


3. Unless you’re Aristotelean.