Such methods were introduced about a 100 year ago by less scientific minded and more spiritual communities such as theosophists, I remember one guru categorically forbidding non-subjective statements to his students. This is good and all that, but can also lead to the other extreme, a form of intellectual laziness where we just agree to disagree that everybody has different tastes and viewpoints and basically thing kind of post-modern laziness, "no truth just different narratives" kind.
My point: put a HEAVY emphasis on the explore-whys. Suppose we want to know what makes books liked. "I like this book, you not, that is okay, tastes are different" type of pomo lazy stuff are IMHO even more useless for this purpose than a categorical "this book is good". At least that leads to "why?" while too much subjectivity not.
I would modify your advice as "move things towards the more subjective or the more objective based on which one seems to lead to more and better whys". Sometimes it requires moving towards the more subjective, sometimes towards the more objective, such as "let's not agree to have different tastes but try to compare what objective quality of the book may have hit us differently?"
This almost seems to be the grown-up form of the childhood game of Why?
"Mom, why is ice cold?"
"Because its temperature is lower than your skin temperature, love."
"Why?"
"Because its molecules are moving slower than yours."
"Why?"
And so on.
I like the idea, but with our current imperfect understanding of the universe, such questions addressing facts must inevitably end with some variation of "Because there are turtles all the way down, love."
I'm not saying that this should be discouraging - rather, that it is good to know where your knowledge ends. Furthermore, each generation (and I use the word "generation" loosely) has succeeded in pushing the turtles down one level more. Maybe one day the game of Why? will actually come to an end...
I don't think it's exploring the depth of knowledge, it's learning to tell maps from the territory, as well as figuring out the limits where the maps stop working.
Why can't it be both? I think that you're right, the technique you describe is good for exploring your own maps, but I also think it seems to work for figuring out where the territory continues but your maps end.
I also think I didn't do a sufficient job of explaining that my "exploring the depths of knowledge" take pertains more to your "The sky is blue" example than your "This book is awful" example (i.e., one that can be answered with fact, rather than opinion.)
"This book is awful" => "I dislike this book" => "I dislike this book because it is shallow and is full of run-on sentences." => I dislike this book because I prefer reading books I find deep and clearly written."
"The sky is blue" => ... => "When I look at the sky, the visual sensation I get is very similar to when I look at a bunch of other objects I've been taught to associate with the color blue."
"Team X lost but deserved to win" => ...
"Being selfish is immoral"
"The Universe is infinite, so anything imaginable happens somewhere"
In general, consider a quick check whether in a given context replacing "is" with "appears to be" leads to something you find non-trivial.
Why? Because it exposes the multiple levels of maps we normally skip. So one might find illuminating occasionally walking through the levels and making sure they are still connected as firmly as the last time. And maybe figuring out where the people who hold a different opinion from yours construct a different chain of maps. Also to make sure you don't mistake a map for the territory.
That is all. ( => "I think that I have said enough for one short post and adding more would lead to diminishing returns, though I could be wrong here, but I am too lazy to spend more time looking for links and quotes and better arguments without being sure that they would improve the post.")