In a post by Kaj Sotala, he introduces the very useful idea of cognitive fusion.

Cognitive fusion is a term from Acceptance and Commitment Therapy (ACT), which refers to a person “fusing together” with the content of a thought or emotion, so that the content is experienced as an objective fact about the world rather than as a mental construct. The most obvious example of this might be if you get really upset with someone else and become convinced that something was all their fault (even if you had actually done something blameworthy too).
In this example, your anger isn’t letting you see clearly, and you can’t step back from your anger to question it, because you have become “fused together” with it and experience everything in terms of the anger’s internal logic.

You can become fused to an emotion, a voice in your head, a political view, and experience it to "just be true". I see this as a similar sort of fusion I hear musician talk about, where after years of practice their instrument begin to feel like a part of their body. They aren't "using their index finger to press the black note on a piano" they are "just playing G". This is analogous to being so caught up in your own anger that your partner is "just wrong and terrible" as opposed to "it sorta looks like you intentionally did something to annoy me and I'm worried about if you'll do this again in the future." (or whatever the actual case is)

Sometimes I think of there being a general fusion process where the brain collapses levels of inference. All of the steps that go into a given physical motion or thought process get compressed into a single dot. The thought process will be experienced as "just true" and the physical motion will be experience as an atomic action available to you. Sometimes you can "uncompress" the chain, and sometimes you can't.

Problems can arise when you fuse to a thought or emotion that doesn't have an accurate view of the world, and you unknowingly take it's broken map as the territory.

Isn't this just "Don't make assumptions"?

Not quite, though it is similar. Assumptions don't really capture the more general fusing process that you can also see with physical movement. "I can't believe that you just assumed you start off on your left foot when making a layup!" Nah, doesn't feel right. But the main reason I prefer to talk in terms of fusion is that "fusion" makes me focus on the process of attaching to something, while "assumptions" makes me focus on the object being attached to.

It's easier to see this difference when the thought being fused to (or the claim being assumed) is "obviously" wrong, or at least obvious to one who isn't fused to it. The assumption frame makes me feel like my work is done when I find the other persons "dumb" assumption. Point it out with a pithy "Checkmate [outgroup]" and move on. The fusion frame leads me to ask "How did they get fused to this in the first place? How might I help them defuse from it?" By focusing on the process of attachment (fusion) I can appreciate how common it is to fuse to something and how hard it can be to defuse. When I focus on the object of attachment (assumption) I'm mostly thinking about just how stupid it is and how I can't believe that anyone would be dumb enough to fall for this, and I most certainly don't believe anything that stupid....

And so it goes. Thinking of some behavior as a "dumb mistake" makes you more likely to not notice when you engage in it. Some thoughts take moments to defuse from. Others take a lifetime. Sometimes you fuse to things, and you'd be wise to learn how it works rather than to ridicule it.

As you may have guessed, later parts of this sequence will talk about what can happen when you fuse with language. For now, just remember what fusion is, and treat it with respect.

"Modern man can't see God because he doesn't look low enough."

-- Carl Jung