Learn English – A person who cannot understand that there may be other perspectives

I'm searching for a word to describe a person who cannot understand that there may be different perspectives. I initially thought of single-minded, but that pertains to determination.

One example of several perspectives could be the difference between a process and an action. Please assume that a process is composed of several actions. A process may be composed of other processes, but these processes would be actions in this greater process.

If you accept my definitions, then depending on the holistic or granularity level, we may consider something a process or an action, depending on whether we are considering its parts or how it's part of a greater thing.

A person may have trouble understanding that it's not true or false that something is a process or an action, it depends on how it relates with everything else from the perspective that we take. From one perspective it's a process, from another it's an action. For some people, things seem to be in one and only one absolute way, which is wrong, IMHO.

Is there a word to describe this kind of problem that some people seem to have?

PD: It's like definition 1b for monism in dictionary.com. But I don't think most people know the word monist.

Here are some other, superficially similar terms that actually mean something quite different. A racist is (approximately) someone who considers other races inferior. A narrow-minded person considers other perspectives wrong or irrelevant, or may not consider them, but is aware that they exist. A monist would not even understand that there is more than one race or multiple perspectives; there is simply one reality, and that's it.