Nothing in Dianetics and Scientology is true for you
Unless you have observed it
And it is true according to your observation.
That is all.
This quote is from a short article by L. Ron Hubbard called "Personal Integrity" which starts out with:
What is personal integrity?
Personal integrity is knowing what you know–
What you know is what you know–
And to have the courage to know and say what you have observed.
And that is integrity.
And there is no other integrity.
So what does this mean?
Let's look at a simple example: You are given a recipe and told that if you follow it exactly then you will produce the best cake you ever tasted. Do you now go around insisting it's the best cake in the world without ever baking it and observing for yourself whether it is or not? Not if you know what Personal Integrity is. If you wanted to retain your own personal integrity then you would bake the cake and take a bite or two so you would know for yourself.
In Scientology you are encouraged to observe the results of applying what you learn in Scientology to life, so that you can see for yourself if it works or not. That is something which people who are used to a religion based solely on faith and belief have a hard time understanding. (Not that there is anything wrong with religions based solely on faith or belief - I'm just using that as a comparison.)
Scientology is a practical religion that is not based on faith or belief. It consists of wisdom that you can actually do something with. E.g., You study how to do a touch assist, then you do one and you see for yourself if the person you helped feels better or not. It is that straightforward.
I recently received a comment that said: I've often been told that Scientologists don't have to believe in various bits of Scientology's writings. I was just wondering which bits you don't believe in.
My guess is that the writer has been talking to people who don't understand Scientology because you don't approach Scientology on the basis of "do I believe that?", you approach it on the basis of "when I use it correctly, does it work?"
I'll give you an example: The first time I did a touch assist I had no idea whether it would work or not. I studied how to do it, I practiced a bit then I went and did one. The woman I was helping had hurt her hand in a printing press. I went through the actions of the touch assist and at one point I asked her how she was doing, because I couldn't see that anything was happening. She said, "Nothing much, I didn't notice any..." then she stopped because she had been moving her hand and it didn't hurt her anymore. She was rather astonished. But the point of the example is that both she and I could observe that the assist had worked because the pain in her hand was gone - which is the end product of an assist.
That is how you are expected to approach Scientology: Study it, use it, see if it works.
So you can see that the comment about "bits you don't believe in" is like asking which bits of physics you don't believe in. Physics contains data that you apply and observe. To say something like, "Oh, I believe in gravity but not magnetism," doesn't make sense because gravity and magnetism are both things you can observe and having observed them you "know" you don't need to "believe".
I hope this helps clarify the meaning of Personal Integrity and clears up any misunderstandings people might have about it.