C.S. Lewis' "Bulverism" is an essay on the foundation of 20th century thought. He begins by stating the two senses in which we exist. The Freudians and Marxians both state that thought is "ideologically tainted" at the source. However, Lewis points out that if this were the case, both the Freudians and Marxians-being systems of thought, just like the rest of us-would both invalidate our thinking and theirs. All they can say is that some thoughts are tainted and others are not. Lewis takes this and points out the problem that we cannot know which thoughts are tainted and which are not.
"Bulverism" is an attempt to point out flaws in reason and thought, by attacking the person's character, rather than stating the flaws in their reason. The danger of falling to Bulverism is that when Bulverism is used, you eliminate any possible discussion of thought and reason.
Lewis encourages us to first show that a man is wrong before you start explaining why he is wrong, for "until Bulverism is crushed, reason can play no effective part in human affairs." He then continues to discuss that God created nature, and that only He is not limited by human understanding of thought and reason.
The essay "Bulverism" is closely related to "Meditation in a Toolshed" in that we must learn to accept humility, so we can accept that we may be wrong. Lewis does not advise people to always believe thoughts of others, but to not reject them merely because of who they are.