Here is a presentation of the difficulties with definition in treating evolution. These difficulties were discovered (a heuristic) in the analytic survey of literature. Later, in the synthetic presentation of theses to develop an academic class on Evolutionism, there will be definitions appropriate to each proposition.

Four general difficulties were discovered with definitions in the treatment of Evolutionism. First, popular definitions can be extended or restricted when applied to science or philosophy, so that terms are used analogously. Second, technical definitions can be diverse in reality and their proponents endorse the effort to keep their theories of evolution distinguished from other theories alleged less accurate. Third, words can be applied equivocally, so that the same word means different things. Fourth, definitions can be lacking.

First, there can be a problem with the extension of the popular concept of evolution.¹ There is a popular meaning, a scientific meaning and a philosophic meaning.² The popular meaning, "change," is an invitation to a number of writers wrongly to allege that those who believe in change, believe in evolution. Also, there is a scientific meaning which states that a very large number of kinds of living things has been derived by means of a tremendously long series of usually very small (perhaps occasionally large) cumulative changes, from very few (perhaps only one) living ancestors. The philosophic meaning, which properly concerns us here, is Evolutionism, the philosophy that holds the complexity of kinds of things is due to the accumulated changes passed on by generation.³

This same analogical problem can arise in connection with such a simple term as "man." There can be a problem with the philosophical definition of man. Even a true universal such as "what a man is" does not settle all that man is, once and for all. The definition of man as a rational animal has been criticized as inadequate and incomplete. This definition of an is essential, good, but never intended to be a complete definition. From the standpoint of completion, much more needs to be said.⁴ There can also be a problem with the scientific definition of man. Man is defined as Homo sapiens sapiens. With the advent of better knowledge of the Neanderthal, some authors are now labeling Neanderthals as Homo sapiens neanderthalensis.⁵ Does the predication of man include all the subspecies under the genus Homo? The obvious use of popular and common terms is not so obvious at all.

Second, technical definitions can be diverse in reality, and the promoters of such technical definitions of evolution seek to have their views of evolution clearly distinguished from other views. There are at least thirty different schools of evolution.⁶ Their definitions of evolution vary considerably and consequently have a different influence on the concept of Evolutionism.⁷ An example of such a difference is the distinction between Darwinism⁸ and the Punctuated Equilibrium Theory⁹ of Evolution; and another example is the distinction between atheistic¹⁰ and theistic¹¹ evolution.

Third, words can be applied equivocally, so that the same word means totally different things.¹²

Fourth, at times, definitions are totally lacking. Reasons for this lack of definitions are the refusal of some scientists who take definitions for granted,¹³ or there may be a disagreement among scientists that prevents definition,¹⁴ or the impediment to definition may be the scientific method itself,¹⁵ or the difficulty may be the philosophy of the scientist.¹⁶ Many of the scientific presentations simply assume the reader knows some popular definition.¹⁷ Darwin himself does not provide ample definitions.¹⁸ Among the Scholastics, it is well known that St. Thomas borrows his language from a diverse number of sources, with the consequence that there is often a need to scrutinize the text of St. Thomas to find not only the personal meaning he applies to the text, but also the personal contribution he makes to the problem he is attempting to solve.¹⁹ Some Neo-Scholastics give definitions, but in a very restricted or even ideosyncratic way.²⁰

Difficulties with definitions of evolution and its elements are common.²¹ Not suprisingly, there are a large variety of meanings attached to the concept "fact of evolution."²² The importance of exact definition is critical in serious dialogue about Evolutionism.