By Michael Ireland, adapted by Marc Chao and Muhamad Alif Bin Ibrahim

A fallacy is a flaw in reasoning that undermines the validity of an argument. This flaw can arise from either the structure of the argument (how its propositions are arranged and connected) or its content (what the propositions actually state). While the form of an argument always matters, informal fallacies specifically deal with errors in how the content of propositions is handled as premises or reasons. This differs from formal fallacies, which we discussed in the previous chapter, and which concern the structural relationships between statements and conclusions.

Informal fallacies typically occur when a premise fails to adequately support a conclusion because it is insufficient, irrelevant, or ambiguous. Unlike formal fallacies, informal fallacies cannot be identified simply by examining the argument’s structure; they require an understanding of the content and context of the premises.

It might surprise you to learn just how common these fallacies are, both in casual conversations and formal debates. For a variety of psychological reasons, fallacious reasoning often seems persuasive, even when it should not be. Part of what makes fallacies so pervasive is their ability to appear convincing on the surface, even though they lack real substance. However, after working through the concepts in previous chapters, you should now be better equipped to recognise and avoid these misleading patterns of reasoning.

Why Study Informal Fallacies?

The list of informal fallacies we will explore in this chapter is not exhaustive. In fact, no single list could ever capture every possible fallacious reason; there are hundreds of known examples, and new ones emerge as our ways of reasoning and communicating evolve. The purpose of this list is not to have you memorise obscure Latin names or classify every bad argument you encounter. Instead, our goal is to help you understand the principles behind why these fallacies fail as reasons so you can recognise them when they appear, regardless of their label.

Organising Informal Fallacies

There is no universally agreed-upon way to classify informal fallacies. Different systems categorise them in various ways. Some may use four categories, while others may organise them differently. Additionally, certain fallacies might comfortably fit into more than one category. For example, a fallacy categorised as insufficient reasoning might also exhibit elements of irrelevance.

Despite these overlaps, organising fallacies into groups can still be helpful because it highlights the general principles that underlie faulty reasoning. For simplicity and clarity, we will focus on a three-category system:

1. Insufficient Reasons – Premises that fail to provide enough support for the conclusion.
2. Irrelevant Reasons – Premises that may seem related but do not actually support the conclusion.
3. Ambiguous Reasons – Premises that are unclear, vague, or open to multiple interpretations.

This three-group approach provides a clear and practical framework for identifying and analysing informal fallacies. Rather than memorising labels, focus on understanding why these types of reasoning fail and how they can be avoided. With these tools, you will be better prepared to assess arguments critically, spot weak reasoning, and construct stronger, more persuasive arguments of your own.