Epistemology is the study of knowledge, and how we come to achieve it. A proper epistemology allows us to gain valid understanding of the world, and identify incorrect ideas. An epistemology based on reason is our means of successfully acquiring knowledge. An irrational epistemology, though, impairs the functioning of the mind. The more irrational it is, the less valid the knowledge one has is.
Since philosophy is a kind of knowledge, an irrational epistemology is the destroyer of a rational philosophy. It is makes it difficult or impossible to get other parts of the philosophy right, since it is prevents the proper functioning of the mind.
Like all misbegotten notions, most irrational epistemological theories or assumptions are not practiced consistently. The result would be an inability to deal with the world. Instead, an irrational epistemology is practiced inconsistently. It impairs the mind when it is used, but it is often ignored allowing limited real use of one's mind.
The following is a list of common epistemological mistakes or flawed systems. It is not an exhaustive list, since there are an infinite number of ways one can be wrong (and only one way to be right).Faith
A Priori Knowledge
Philosopher's Deduction Fallacy
Fallacy of the Second Standard