How to Use the Taxonomy
Beginning with Aristotle, the first logician to name fallacies, most logicians who have studied fallacies have classified them into types. Aristotle classified his list of fallacies into two types:
Subsequent logicians have usually extended Aristotle's classification by subdividing the second, non-linguistic, category into, for instance, fallacies of relevance and fallacies of presumption. However, most such classifications have remained relatively "flat", with all fallacies on the same level, but a flat classification does not do justice to the complexity of the logical relations between different fallacies.
The Taxonomy is a tree-like structure that classifies all of the fallacies in these files by the subfallacy relation. A subfallacy, which is a specific version of a more general fallacy, has whatever features the more general fallacy has, together with specific features which set it apart and make it worth naming in its own right. For example, instead of grouping together "fallacies of relevance", there is one most general such fallacynamely, Red Herringand all fallacies of relevance are subfallacies of it. Red Herring is itself a subfallacy of Informal Fallacy, which is a subfallacy of the most general logical fallacy of all: Logical Fallacy. Logical Fallacy is, thus, the top node of the Taxonomy, for every fallacy in the Taxonomy is a subfallacy of it. The subfallacy relationship is like a tree with a trunk―Logical Fallacy―which branches until it reaches leaves, that is, fallacies which have no subfallacies―for example, Appeal to Celebrity. In the Taxonomy as pictured, the tree is lying on its side with the trunk to the left and the leaves to the right, connected by black branches which represent the subfallacy relation.
The Taxonomy uses the following color-coding: The most general fallacy, Logical Fallacy, is white. Its immediate subfallacies split into the red, blue, and green colors that constitute white light. Formal fallacies are colored red, and informal ones blue. This color scheme is entirely arbitrary and has no meaning itself, except to visually distinguish the types of fallacy―in fact, it has the consequence that the informal fallacy Red Herring is blue! The odd man out is Loaded Question, which is colored green. Also, the colors fade, becoming more muted as one proceeds from more general fallacies on the left to more specific ones on the right.
Some fallacies―such as Ambiguous Middle―are purple or violet―that is, a blend of red and blue―because they have both formal and informal aspects and are, therefore, subfallacies of both Formal and Informal Fallacy. This means, of course, that formal and informal are not disjoint categories, as one might expect. This is because the fallacies in common to both categories have a formal and a linguistic dimension. For instance, Ambiguous Middle is a type of Four-Term Fallacy, thus violating the formal rules of categorical syllogisms; but it is also a type of Equivocation, an informal fallacy, since a single word stands for two of the four terms.
The Taxonomy is more useful than the alphabetical index for studying the logical relationships between fallacies. To understand an individual fallacy, it may be helpful to move upward in the Taxonomy―that is, to the left―in order to understand the more general fallacy of which it is a subfallacy. Also, moving downward―that is, to the right―can help in understanding a general fallacy through seeing more specific versions of it. Some individual fallacies―such as Wishful Thinking―are leaves on more than one branch of the Taxonomy, because they are subfallacies of more than one type of fallacy.
In addition, fallacies that are subfallacies of the same general fallacy are like siblings, since they share the same parent. So, it may help to compare and contrast a fallacy with its siblings. As with human siblings, the likeness between sibling fallacies is stronger in some cases than in others. For instance, the causal fallacies Post Hoc and Cum Hoc are more similar to each other than they are to their other siblings, the Regression and Texas Sharpshooter fallacies. In the Taxonomy, this strong sibling relationship is indicated by a thicker, similarly-colored line connecting the two fallacies.
Another use for the Taxonomy is in finding a fallacy whose name you do not know, but you do know what general type of mistake you are looking for. Start with a general fallacy, and "drill down" into the Taxonomy―that is, moving to the right―until you find what you're looking for.
Acknowledgment: Thanks to Donna Chantler for pointing out some broken links.