Not sure what you mean about assuming a self-sustaining thought.
The point is that you can never experience anything outside your own point of view, which means that you can never know if anything outside of your own personal experience is at all separate or independent of it.
Sure, you can “trust” others when they say something to you, but you can never experience anything from someone else’s perspective (unless you could become them, but then you’d stop being you). All experience is subjective. We can all try to agree on what something (a physical process) means, but that is only an agreement, it is not “true reality”. So in that sense, you can never know what “true reality” is, except for whatever you subjectively experience.
After all, the only thing that we can be certain of is our own (subjective) experience.